The influence of protonation reactions between poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a thiadiazolo3,4‐cpyridine small‐molecule donor are reported; these result in ...poor solar‐cell performance due to a barrier for charge extraction. The use of a NiOx contact eliminates such deleterious chemical interactions and results in substantial improvements in open‐circuit voltage, fill factor, and an increased power conversion efficiency from 2.3% to 5.1%.
We report the fabrication and measurement of solar cells approaching a power conversion efficiency of 3.2% using a low band gap conjugated polymer ...poly2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta2,1-b;3,4-b′dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) and CdSe nanoparticles. These devices exhibit an external quantum efficiency (EQE) of >30% in a broad range of 350−800 nm with a maximum EQE of 55% in a range of 630−720 nm. We also present certified device efficiencies of 3.13% under AM 1.5 illumination.
Solvent additives such as diiodooctane (DIO) are becoming ubiquitous in processing high performance organic photovoltaic (OPV) active layers. Here, we investigate the effects of DIO on the long-term ...stability of the active layer by studying the photodegradation under ambient white light illumination of the polymer PTB7-Th in pure polymer thin films and in blend films with PC71BM. Using X-ray fluorescence, we directly detect iodine in the active layer films, indicating the presence of residual DIO after casting from solution. Additionally, we show that this residual DIO dramatically decreases the photostability of the active layer. Structural changes in the films upon illumination are probed with grazing-incidence wide-angle X-ray scattering (GIWAXS). FTIR spectroscopy is used to monitor chemical changes in the polymer structure during irradiation in the presence of DIO. Furthermore, we demonstrate that film treatment either with high vacuum (10–8 Torr) for 60 min or with a high-temperature thermal anneal at 175 °C for 30 min removes residual DIO from the film and delays photodegradation. Therefore, when processing polymer solar cells with DIO-containing solutions, it is imperative to remove any trace amounts of DIO from deposited films.
The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiO x ) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) ...are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O2-plasma treatment of the as-deposited NiO x films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni2O3, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O2-plasma treated NiO x films are compared using both electron-only and BHJ devices. O2-plasma-treated NiO x interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiO x interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiO x films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime.
We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work‐function of the hole collecting indium‐tin oxide (ITO) contact, modified with a ...solution‐processed nickel oxide (NiOx) hole‐transport layer (HTL), is matched to the ionization potential of the donor material in a bulk‐heterojunction solar cell. Addition of the NiOx HTL to the hole collecting contact results in a power conversion efficiency (PCE) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL on ITO. The impact of these NiOx films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiOx HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiOx HTL is shown to provide superior contact properties by increasing the ITO/NiOx contact work‐function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiOx interface with the poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothidiazole) (PCDTBT):6,6‐phenyl‐C61 butyric acid methyl ester PCBM and 6,6‐phenyl‐C71 butyric acid methyl ester (PC70BM) active layer. A high work‐function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance.
Enhanced efficiency and stability of a high performance organic solar cell is demonstrated when the hole collecting indium‐tin oxide contact is modified with solution‐processed nickel oxide and the high work‐function contact is matched with the donor ionization potential. We use photoemission spectroscopy to characterize the band edge positions in the contact layer, a multilayer matrix optical model and a diode model to explain the improvements in contact quality in comparison to the hole‐transport layer, polyethylene dioxythiophene:polystyrene sulfonate.
Organic semiconductor-based photovoltaic devices offer the promise of a low-cost photovoltaic technology that could be manufactured via large-scale, roll-to-roll printing techniques. Existing organic ...photovoltaic devices have currently achieved solar power conversion efficiencies greater than 3%. Although encouraging, the reasons higher efficiencies have not been achieved are poor overlap between the absorption spectrum of the organic chromophores and the solar spectrum, non-ideal band alignment between the donor and acceptor species, and low charge carrier mobilities resulting from the disordered nature of organic semiconductors. To address the latter issues, we are investigating the development of nanostructured oxide/conjugated polymer composite photovoltaic (PV) devices. These composites can take advantage of the high electron mobilities attainable in oxide semiconductors and can be fabricated using low-temperature solution-based growth techniques. Additionally, the morphology of the composite can be controlled in a systematic way through control of the nanostructured oxide growth. ZnO nanostructures that are vertically aligned with respect to the substrate have been grown. Here we discuss the fabrication of such nanostructures and present results from ZnO nanofiber/poly(3-hexylthiophene) (P3HT) composite PV devices. The best performance with this cell structure produced an open circuit voltage (
V
oc) of 440 mV, a short circuit current density (
J
sc) of 2.2 mA/cm
2, a fill factor (FF) of 0.56, and a conversion efficiency (
η) of 0.53%. Incorporation of a blend of P3HT and (6,6)-phenyl C
61 butyric acid methyl ester (PCBM) into the ZnO nanofibers produced enhanced performance with a
V
oc of 475 mV,
J
sc of 10.0 mA/cm
2, FF of 0.43, and
η of 2.03%. The power efficiency is limited in these devices by the large fiber spacing and the reduced
V
oc.
A spin‐cast method is presented for the formation of phosphonic acid functionalized small molecule layers on solution‐processed ZnO substrates for use as electron collecting interlayers in organic ...photovoltaics. Phosphonic acid interlayers modify the ZnO work function and the charge carrier injection barrier at its interface, resulting in systematic control of V OC in inverted bulk heterojunction solar cells. Surface modification is shown to moderate the need for UV light‐soaking of the ZnO contact layers. Lifetime studies (30 days) indicate stable and improved OPV performance over the unmodified ZnO contact, which show significant increases in charge extraction barriers and series resistance. Results suggest that enhanced stability using small molecule modifiers is due to partial passivation of the oxide surface to molecular oxygen adsorption. Surface passivation while maintaining work function control of a selective interlayer can be employed to improve net efficiency and lifetime of organic photovoltaic devices. The modified cathode work function modulates V
OC via static energetic barriers and modulates contact conductivity by creating reversible and irreversible S‐shape current‐voltage characteristics as a result of kinetic barriers to charge transport.
Deposition of benzyl phosphonic acids and alkanethiol self‐assembled monolayers improve initial device performance, and have beneficial effect at mitigating the light‐soaking effect present after aging inverted architecture organic bulk heterojunction devices incorporating ZnO contact layers in air. The effect of a kinetic/transport barrier and a static energetic barrier resulting in formation of S‐shaped J–V curves is isolated.
The role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (VOC) in bulk heterojunction organic photovoltaics is examined for ...poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and 6,6‐phenyl‐C71 butyric acid methyl ester (PC71BM) solar cells. In the absence of a charge selective, electron blocking contact, systematic variation of the work function of the contact directly dictates the VOC, as defined by the energetic separation between the relative Fermi levels for holes and electrons, with little change in the observed dark saturation current, J0. Improving the charge selectivity of the contact through an increased barrier to electron injection from the fullerene in the blend into the hole contact results in a decreased reverse saturation current (decreased J0 and increased shunt resistance, RSH) and improved VOC. Based on these observations, we provide a set of contact design criteria for tuning the VOC in bulk heterojunction organic photovoltaics.
Contact work function and thermodynamic selectivity both play key roles in determining the open circuit voltage in PCDTBT:PC71BM OPVs. Systematically increasing the work function of the contact increases the VOC; adding a selective contact with the same work function further increases the VOC due to a change in the space charge region at the active layer/contact interface.
We present detailed investigations of the fabrication and characterization of photovoltaic devices consisting of poly(3-hexylthiophene) (P3HT) intercalated into a mesoporous structure of ZnO ...nanofibers. ZnO nanofibers were grown via a low-temperature hydrothermal route from a solution of zinc nitrate precursor. P3HT was spin-coated on top of the structure, and intercalation into the voids between the nanofibers was induced with annealing. A silver electrode was used as the top contact. Spin-coating, storage, and testing of the device were performed in air. We discuss the effects of atmosphere and ZnO nanofiber morphology on device performance. Optimized nanofiber devices exhibited a 4-fold increase in the short circuit current (2.17 mA/cm2) as compared to that of a planar ZnO−P3HT bilayer device (0.52 mA/cm2) as a result of the increased donor−acceptor interfacial area. The efficiency of the nanofiber based device under 1 sun-simulated solar illumination was 0.53% and was found to increase at higher incident light intensities, reaching a value of 0.61% at 2.5 suns. Additionally, we found that for these devices fabrication in and exposure to air is required to obtain good diode characteristics. We also show that the morphology of the ZnO nanostructures in the nanocomposite directly impacts device performance. Treatment of the ZnO surface using surfactants increased the open circuit voltage at the expense of the short circuit current; however, there was little effect on overall device efficiency. Because of the inverted geometry of this device that allowed for the use of a silver top contact, the device was not susceptible to oxidative degradation when stored in the dark.
The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiO{sub x}) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics ...(OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O₂-plasma treatment of the as-deposited NiO{sub x} films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni₂O₃, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O₂-plasma treated NiO{sub x} films are compared using both electron-only and BHJ devices. O₂-plasma-treated NiO{sub x} interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiO{sub x} interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiO{sub x} films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime.
