Introduction: Solar Paint
A particular paint which produces direct electricity from the sunlight.
Organic photovoltaics (OPVs) offer enormous potential as inexpensive coatings capable of generating electricity directly from sunlight. These polymer blend materials can be printed at high speeds across large areas using roll-to-roll processing techniques, creating the tantalising vision of coating every roof and other suitable building surface with low-cost photovoltaics.
Step 1: Synthesis of NPs Via the Miniemulsion Process
The nanoparticle fabrication method utilises ultrasound energy delivered via an ultrasound horn inserted into the reaction mixture to generate a miniemulsion (Figure above). The ultrasound horn makes the formation of sub-micrometre droplets possible by applying high shear force. A liquid aqueous surfactant-containing phase (polar) is combined with an organic phase of polymer dissolved in chloroform (non-polar) to generate a macroemulsion, then ultrasonicated to form a miniemulsion. The polymer chloroform droplets constitute the dispersed phase with an aqueous continuous phase. This is a modification of the usual method for generating polymer nanoparticles where the dispersed phase was liquid monomer.
Immediately after miniemulsification, the solvent is removed from the dispersed droplets via evaporation, leaving polymer nanoparticles. The final nanoparticle size can be varied by changing the initial concentration of surfactant in the aqueous phase.
Step 2: Synthesis of NPs Via Precipitation Methods
As an alternative to the miniemulsion approach, precipitation techniques offer a simple route to the production of semiconducting polymer nanoparticles via the injection of a solution of active material into a second solvent of poor solubility.
As such, the synthesis is quick, does not use surfactant, requires no heating (and therefore, no prefabrication annealing of the nanoparticles) in the nanoparticle synthesis phase and can readily be scaled up for the large-scale synthesis of material. In general, the dispersions have been shown to have lower stability and exhibit a compositional change upon standing due to preferential precipitation of particles of differing composition. However, the precipitation approach does offer the opportunity for inclusion of the nanoparticle synthesis as part of an active printing process, with particles being generated as and when required. Furthermore, Hirsch et al. have shown that by successive solvent displacement, it is possible to synthesise inverted core-shell particles where the structural arrangement is counter to the inherent surface energies of the materials.
Step 3: The PFB:F8BT Nanoparticulate Organic Photovoltaic (NPOPV) Material System
Early measurements of the power conversion efficiency of PFB:F8BT nanoparticle devices under solar illumination reported devices with a Jsc = 1 × 10 −5 A cm^−2 and Voc = 1.38 V , which (assuming a best estimate unannealed fill factor (FF) of 0.28 from bulk blend devices) corresponds to a PCE of 0.004%.
The only other photovoltaic measurements of PFB:F8BT nanoparticle devices were external quantum efficiency (EQE) plots. Multilayered photovoltaic devices fabricated from PFB:F8BT nanoparticles, which demonstrated the highest power conversion efficiencies observed for these polyfluorene nanoparticle materials.
This increased performance was achieved through the control of the surface energies of the individual components in the polymer nanoparticle and the post-deposition processing of the polymer nanoparticle layers. Significantly, this work showed that the fabricated nanoparticulate organic photovoltaic (NPOPV) devices were more efficient than the standard blend devices (Figure later).
Step 4: Figure
Comparison of the electrical characteristics of nanoparticle and bulk heterojunction devices. (a) Variation of current density vs. voltage for a five-layer PFB:F8BT (poly(9,9-dioctylfluorene-co-N,N'-bis(4-butylphenyl)-N,N'-diphenyl-1,4-phenylenediamine) (PFB); poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT)) nanoparticulate (filled circles) and a bulk heterojunction (open circles) device; (b) Variation of external quantum efficiency (EQE) vs. wavelength for a five-layer PFB:F8BT nanoparticulate (filled circles) and a bulk heterojunction (open circles) device. Also shown (dashed line) is the EQE plot for the nanoparticulate film device.
The effect of Ca and Al cathodes (two of the most common electrode materials) in OPV devices based on polyfluorene blend aqueous polymer nanoparticle (NP) dispersions. They showed that PFB:F8BT NPOPV devices with Al and Ca/Al cathodes exhibit qualitatively very similar behaviour, with a peak PCE of ~0.4% for Al and ~0.8% for Ca/Al, and that there is a distinct optimized thickness for the NP devices (next Figure). The optimal thickness is a consequence of the competing physical effects of the repair and filling of defects for thin films [32,33] and the development of stress cracking in thick films.
The optimal layer thickness in these devices corresponds to the critical cracking thickness (CCT) above which stress cracking occurs, resulting in low shunt resistance and a reduction in device performance.
Step 5: Figure
Variation of power conversion efficiency (PCE) with the number of deposited layers for PFB:F8BT nanoparticulate organic photovoltaic (NPOPV) devices fabricated with an Al cathode (filled circles) and a Ca/Al cathode (open circles). Dotted and dashed lines have been added to guide the eye. An average error has been determined based upon the variance for a minimum of ten devices for each number of layers.
So, F8BT devices enhances the exciton dissociation relative to the corresponding BHJ structure. Moreover, the use of a Ca/Al cathode results in the creation of interfacial gap states (Figure later), which reduce the recombination of charges generated by the PFB in these devices and restores open circuit voltage to the level obtained for an optimized BHJ device, resulting in a PCE approaching 1%.
Step 6: Figure
Energy level diagrams for PFB:F8BT nanoparticles in the presence of calcium. (a) Calcium diffuses through the nanoparticle surface; (b) Calcium dopes the PFB-rich shell, producing gap states. Electron transfer occurs from calcium producing filled gap states; (c) An exciton generated on PFB approaches the doped PFB material (PFB*), and a hole transfers to the filled gap state, producing a more energetic electron; (d) Electron transfer from an exciton generated on F8BT to either the higher energy PFB lowest unoccupied molecular orbital (LUMO) or the filled lower energy PFB* LUMO is hindered.
NP-OPV devices fabricated from water-dispersed P3HT:PCBM nanoparticles that exhibited power conversion efficiencies (PCEs) of 1.30% and peak external quantum efficiencies (EQE) of 35%. However, unlike the PFB:F8BT NPOPV system, the P3HT:PCBM NPOPV devices were less efficient than their bulk heterojunction counterparts. Scanning transmission X-ray microscopy (STXM) revealed that the active layer retains a highly structured NP morphology and comprises core-shell NPs consisting of a relatively pure PCBM core and a blended P3HT:PCBM shell (next Figure). However, upon annealing, these NPOPV devices undergo extensive phase segregation and a corresponding decrease in device performance. Indeed, this work provided an explanation for the lower efficiency of the annealed P3HT:PCBM OPV devices, since thermal processing of the NP film results in an effectively “over-annealed” structure with gross phase segregation occurring, thus disrupting charge generation and transport.
Step 7: Summary of NPOPV Performance
A summary of the performance of NPOPV devices reported over the past few years is presented in
Table. It is clear from the table that the performance of NPOPV devices has increased dramatically, with a rise of three orders of magnitude.
Step 8: Conclusions and Future Outlook
The recent development of water-based NPOPV coatings represents a paradigm shift in the development of low-cost OPV devices. This approach simultaneously provides control of morphology and eliminates the need for volatile flammable solvents in device production; two key challenges of current OPV device research. Indeed, the development of a water-based solar paint offers the tantalising prospect of printing large-area OPV devices using any existing printing facility. Moreover, it is increasingly recognised that the development of a water-based printable OPV system would be highly advantageous and that the current material systems based on chlorinated solvents are not suitable for commercial scale production. The work described in this review shows that the new NPOPV methodology is generally applicable and that NPOPV device PCEs can be competitive with devices built from organic solvents. However, these studies also reveal that, from a materials point of view, NPs behave completely differently from polymer blends spun from organic solvents. Effectively, the NPs are a completely new material system, and as such, the old rules for OPV device fabrication that have been learnt for organic-based OPV devices no longer apply. In the case of NPOPVs based on polyfluorene blends, the NP morphology results in a doubling of the device efficiency. However, for polymer:fullerene blends (e.g., P3HT:PCBM and P3HT:ICBA), morphology formation in the NP films is highly complex, and other factors (such as core diffusion) can dominate, resulting in unoptimised device structures and efficiencies. The future outlook for these materials is extremely promising, with device efficiencies having increased from 0.004% to 4% in less than five years. The next stage of development will involve understanding the mechanisms that determine NP structure and NP film morphology and how these can be controlled and optimised. To date, the ability to control the morphology of OPV active layers on the nanoscale has yet to be realized. However, recent work demonstrates that the application of NP materials may allow this goal to be achieved.