Orientation management of α-sexithiophene layer for the application in organic photovoltaic devices

Management of α-sexithiophene (6T) molecular orientation is a key issue to improve the performance of organic photovoltaic (OPV) devices based on 6T thin films, which act as p-type semiconductor layers; however, it is difficult to produce 6T layers with the molecules lying on smooth surfaces, including oxide covered substrates. We have succeeded in orienting the 6T molecules on oriented conductive polythiophene (PT) films, parallel to the oriented PT molecular axis; i.e. parallel to the substrate, by evaporating 6T in vacuum. Here, we reported planar heterojunction (PHJ) OPV devices consisting of 6T films with molecular orientation parallel to the substrate. The orientation of 6T led to improvement of the power conversion efficiency (PCE) in these PHJ OPVs. The average PCE of PHJ OPVs with oriented PT was 2.8 times higher than that without PT. The PCE and the maximum value of the short-circuit photocurrent action spectra of the OPV with the PT was larger under irradiation of polarized light parallel to the 6T molecular axis than that orthogonal to the axis. On the other hand, the PCE and the action spectra of the OPV without the PT did not show the response to polarized light.     

Electric conduction of PbBr-based layered perovskite organic–inorganic superlattice having carbazole chromophore-linked ammonium molecule as an organic layer

We have successfully prepared thin films of PbBr-based layered perovskite having hole-transporting carbazole chromophore-linked ammonium molecules as an organic layer by a simple spin-coating from the N,N-dimethylformamide solution in which the stoichiometric amount of lead bromide and carbazole-linked ammonium bromides was dissolved. Their X-ray diffraction profiles exhibited that their layer structure formed (0 0 n)-orientation, where c-axis is perpendicular to the substrate plane. Their layer structure depended on the alkyl chain length of ammonium molecules. When methylene length of C₅H₁₀ was employed in the carbazole-linked ammonium molecules, highest orderliness of the layer structure was attained; higher-order X-ray diffraction peaks were observed in the layered perovskite films. In the layered pervskite film, in-plane conduction, namely conduction in the direction of the stacking of carbazole chromophore, was measured. For comparison, conductivity of poly(N-vinylcarbazole) (PVCBz) thin film was also measured. The conductivity of the layered perovskite thin film (1.8 × 10^?10 Scm^?1 at 303 K) was about three order of magnitude larger than that of the PVCBz thin film (5.3 × 10^?14 Scm^?1 at 303 K). Despite the much higher conductivity of the layered peroskite thin film, the activation energy of the conductivity of the layered perovskite thin film (1.44 eV) was about 2.4 times larger than that of the PVCBz thin film (0.61 eV). This phenomenon is probably due to difference in film morphology through considering the results of AFM observation.     

Small molecule donors with different conjugated π linking bridges: Synthesis and photovoltaic properties

Three small molecule (SM) donors, namely B-T-CN, B-TT-CN and B-DTT-CN, with different π conjugated bridges were synthesized in this research. Interestingly, with the conjugated fused rings of the π linking bridge increasing, the SM HOMO levels exhibit a decline tendency with –5.27 eV for B-T-CN, –5.31 eV for B-TT-CN and –5.40 eV for B-DTT-CN. After blending the SM donors with the fullerene acceptor PC₇₁BM, the all SM organic solar cells (OSCs) achieved high V_ocs of 0.90 to 0.96 V. However, the phase separation morphology and molecule stacking are also unexpectedly changed together with the enhancement of conjugated degree of π bridges, resulting in a lower power conversion efficiency (PCE) for the B-DTT-CN:PC₇₁BM device. Our results demonstrate and provide a useful way to enhance OSC V_oc and the morphology needs to be further optimized.     

Improved power conversion efficiency by insertion of RGO–TiO2 composite layer as optical spacer in polymer bulk heterojunction solar cells

We report that the power conversion efficiency (PCE) can be enhanced in polymer bulk heterojunction solar cells by inserting an interfacial electron transporting layer consisting of pristine TiO₂ or reduced graphene oxide–TiO₂ (RGO–TiO₂) between the active layer and cathode Al electrode. The enhancement in the PCE has been analyzed through the optical absorption, current–voltage characteristics under illumination and estimation of photo-induced charge carrier generation rate. It was found that either TiO₂ or RGO–TiO₂ interfacial layers improve the light harvesting, as well as the charge extraction efficiency, acting as a blocking layer for holes, and also reducing charge recombination. The combined enhancement in light harvesting property and charge collection efficiency improves the PCE of the organic solar cell up to 4.18% and 5.33% for TiO₂ and RGO–TiO₂ interfacial layer, respectively, as compared to a value of 3.26% for the polymer solar cell without interfacial layer.     

Inorganic perovskite/organic tandem solar cells with efficiency over 20%

The rapid development of perovskite solar cells is beyond our imagination.The power conversion efficiency(PCE)of organic-inorganic hybrid perovskite solar cells has reached 25.5%(https://www.nrel.gov/pv/cell-efficiency.html).However,the unsatisfactory stability of hybrid perovskites is an obstacle for their commercialization,which results from the volatile and hygroscopic organic cations[1].     

Tandem small molecule organic photovoltaic cells with broad spectral response up to 1 μm and a high open-circuit voltage

We report a series-connected small molecule tandem photovoltaic cell utilizing two donors with complementary photovoltaic characteristics, lead phthalocyanine (PbPc) in the front subcell and boron subphthalocyanine chloride (SubPc) in the back subcell, to achieve both near infrared (NIR) response up to 1 μm and high open-circuit voltage (V_OC) of more than 1.5 V in the same device. We find that the C₆₀ layer thickness in the front subcell has a critical impact on the overall optical structure and photovoltaic performance of the tandem device. By combining transfer matrix calculations with subcell-selective spectral measurements, we are able to tune the optical field distribution inside the active layers and increase the photocurrent outputs from both subcells, leading to EQE > 30% over the wavelength range 400 nm < λ < 900 nm. This optimized tandem cell exhibits J_SC = (5.5 ± 0.1) mA/cm², fill factor = 0.54, V_OC = 1.53 V, and a power conversion efficiency of (4.5 ± 0.2)%.     

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

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

Study of the light-induced degradation of tandem α-Si:H/μc-Si:H photovoltaic converters

The photo-induced degradation of tandem α-Si:H/μc-Si:H photovoltaic converters with an initial efficiency of 10.4% under light flux densities of 1 and 10 kW m^?2 (AM1.5G) is studied. It is shown that the stabilized state is reached after 500 h of exposure to the standard light-flux density and after 300 min at a flux 10 times higher in density. In both cases, the efficiency decreases by 1.2–1.4 abs. %. The experimentally measured spectral and current-voltage characteristics of the photovoltaic converters are used to determine the nonequilibrium carrier lifetimes and to calculate variation dependences of the dangling-bond concentration in i-α-Si:H and i-μc-Si:H layers. The dependences are approximated in terms of the floating-bond model. The calculated dangling-bond concentrations after various exposure times are used to simulate the dependences of the photovoltaic-converter parameters on light exposure. The results obtained show good coincidence between the simulated degradation rates of the current and efficiency of a tandem photovoltaic cell and the experimental data.     

MoO3–Au composite interfacial layer for high efficiency and air-stable organic solar cells

Efficient and stable polymer bulk-heterojunction solar cells based on regioregular poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PC₆₁BM) blend active layer have been fabricated with a MoO₃–Au co-evaporation composite film as the anode interfacial layer (AIL). The optical and electrical properties of the composite MoO₃–Au film can be tuned by altering the concentration of Au. A composite film with 30% (weight ratio) Au was used as the AIL and showed a better performance than both pure MoO₃ and PEDOT:PSS as AIL. The surface morphology of the MoO₃–Au composite film was investigated by atomic force microscopy (AFM) and showed that the originally rough ITO substrate became smooth after depositing the composite film, with the root mean square roughness (RMS) decreased from 4.08 nm to 1.81 nm. The smooth surface reduced the bias-dependent carrier recombination, resulting in a large shunt resistance and thus improving the fill factor and efficiency of the devices. Additionally, the air stability of devices with different AILs (MoO₃–Au composite, MoO₃ and PEDOT:PSS) were studied and it was found that the MoO₃–Au composite layer remarkably improved the stability of the solar cells with shelf life-time enhanced by more than 3 and 40 times compared with pure MoO₃ layer and PEDOT:PSS layer, respectively. We argue that the stability improvement might be related with the defect states in MoO₃ component.     

Method for studying the light-induced degradation of α-Si:H/μc-Si:H tandem photovoltaic converters under increased illuminance

An experimental installation and the procedure for its use in accelerated tests of thin-film α-Si:H/μc-Si:H photovoltaic converters with dimensions of up to 100 × 100 mm on light-induced degradation under an increased illumination level (up to 10 kW/m²) are described. Estimates of the levels of photoinduced degradation of photovoltaic converters, obtained by conventional and developed procedures, are compared and it is demonstrated that the procedure for studying the photoinduced degradation at an increased illumination level can make the test duration 100 times shorter while providing fully adequate assessment of the stability of photovoltaic converters based on amorphous and microcrystalline silicon.     

Effect of UV light irradiation on photovoltaic characteristics of inverted polymer solar cells containing sol–gel zinc oxide electron collection layer

An inverted organic bulk-heterojunction solar cell containing a zinc oxide (ZnO) based electron collection layer with a structure of ITO/ZnO/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene): poly(4-styrene sulfonic acid)/Au (ZnO cell) was fabricated. We examined the relationship between the heating temperature of the ZnO layer and the device performance under irradiation by simulated sunlight while cutting the UV light. The effects of the UV light contained in simulated sunlight were investigated by photocurrent–voltage (I–V) and alternating current impedance spectroscopy (IS) measurements. When the ZnO cells were irradiated with simulated sunlight, they exhibited a maximum power conversion efficiency (PCE) of over 3%, which hardly varied with the heating temperature of ZnO layers treated at 250 °C, 350 °C, and 450 °C. In contrast, when the ZnO cells were irradiated with simulated sunlight without UV content, their photovoltaic characteristics were very different. In the case of the cell with ZnO prepared by heating at 250 °C, PCE of 2.7% was maintained even under continuous irradiation with simulated sunlight without UV. However, for the cells with ZnO prepared by heating at 350 °C and 450 °C, the shapes of the I–V curves changed with the UV-cut light irradiation time, accompanying an increase in their series resistance. Overall, after UV-cut light irradiation for 1 h, the PCE of the cell with ZnO prepared by heating at 350 °C decreased to 1.80%, while that of the cell with ZnO prepared by heating at 450 °C fell to 1.35%. The photo IS investigations suggested that this performance change was responsible for the formation of charge-trapping sites at the ZnO/PCBM:P3HT interface which act as recombination centers for photo-produced charges in the PCBM:P3HT layer.     

A crystalline D-π-A organic small molecule with naphtho[1,2-b:5,6-b′]dithiophene-core for solution processed organic solar cells

In this work, we have designed and synthesized a new naphtho[1,2-b:5,6-b′]dithiophene-containing enlarged π-conjugated donor–acceptor (D–A) small molecule, NDT(TTz)₂, for use in solution-processed organic photovoltaics. NDT(TTz)₂, which contains a thiophene-bridged naphtho[1,2-b:5,6-b′]dithiophene as the central fused core and triphenylamine-flanked thiophene thiazolothiazole as a spacer, was synthesized via sequential Suzuki and Stille coupling reactions. The thermal, physiochemical, and electrochemical properties of NDT(TTz)₂ have been evaluated by differential scanning calorimetry, thermogravimetry, UV–Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction, and cyclic voltammetry. As desired for photovoltaic applications, NDT(TTz)₂ possesses good solubility, thermal stability, and a well-ordered, π–π stacked, crystallinity. The optical band gap and HOMO level of NDT(TTz)₂ were determined to be 2.0 eV and −5.23 eV, respectively. In addition to organic thin film transistor studies, application of NDT(TTz)₂ to preliminary photovoltaic devices has also been investigated by fabricating solution-processed bulk heterojunction solar cells together with PC₇₁BM in a typical layered device structure, ITO/PEDOT:PSS/NDT(TTz)₂:PC₇₁BM/LiF/Al. Without extensive optimization of the device, NDT(TTz)₂ in these devices shows a maximum power conversion efficiency of 1.44% under AM 1.5 illumination at a 100 mW/cm² intensity.     

The impurity photovoltaic (IPV) effect in wide‐bandgap semiconductors: an opportunity for very‐high‐efficiency solar cells?

Following recent progress in the study of limiting efficiencies of photovoltaic devices with multiple energy levels, we suggest using the impurity photovoltaic (IPV) effect in wide‐bandgap semiconductors as a means to achieve very‐high‐efficiency solar cells. We discuss the requirements for a high‐efficiency IPV device and review some of the material systems that could be used. As a case study, we investigate theoretically β‐SiC IPV solar cells with a model based on a modified Shockley–Read–Hall theory. The high‐efficiency potential is confirmed and the important issues for implementation are presented and discussed. Copyright © 2002 John Wiley &; Sons, Ltd.