Concept: Fill factor
A periodically aligned Pt nanocup array is employed as a counter electrode in dye-sensitized solar cells (DSSCs). The Pt nanocup array enhances the catalytic area, which results in a greater fill factor (FF) and higher short-circuit current (J(sc) ). A higher density of nanocups is preferable for the performance of DSSCs.
A simple solution-processing method was employed to fabricate panchromatic mp-TiO2/CH3NH3PbI3/P3HT-MWNT/Au solar cells. MWNTs in a P3HT-MWNT composite acted as efficient nanostructured charge transport tunnels and induce crystallization of P3HT, hence significantly enhancing the conductivity of the composite. The fill factor of the hybrid solar cells was greatly enhanced by 26.7%.
Morphology control of solution coated solar cell materials presents a key challenge limiting their device performance and commercial viability. Here we present a new concept for controlling phase separation during solution printing using an all-polymer bulk heterojunction solar cell as a model system. The key aspect of our method lies in the design of fluid flow using a microstructured printing blade, on the basis of the hypothesis of flow-induced polymer crystallization. Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes. The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation. We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.
Formamidinium-lead-iodide (FAPbI3)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap. Record-performance FAPbI3-based solar cells have relied on fabrication via the sequential-deposition method; however, these devices exhibit unstable output under illumination due to the difficulty of incorporating cesium cations (stabilizer) in sequentially deposited films. Here we devise a perovskite seeding method that efficiently incorporates cesium and beneficially modulates perovskite crystallization. First, perovskite seed crystals are embedded in the PbI2 film. The perovskite seeds serve as cesium sources and act as nuclei to facilitate crystallization during the formation of perovskite. Perovskite films with perovskite seeding growth exhibit a lowered trap density, and the resulting planar solar cells achieve stabilized efficiency of 21.5% with a high open-circuit voltage of 1.13 V and a fill factor that exceeds 80%. The Cs-containing FAPbI3-based devices show a striking improvement in operational stability and retain 60% of their initial efficiency after 140 h operation under one sun illumination.
To apply CuInSe₂ (CIS)-based printable solar batteries; an aqueous phase synthesis method of Cu-In (CI) alloy nanoparticles is studied. Metal complexes in the original solution are restricted to homogenized species by utilizing calculations. For example; [(Cu2+)(ASP2-)₂] [ASP: the “body (C₄H₅O₄N)” of aspartic acid (C₄H₇O₄N)] is predominant in the pH 6-13 region (CASP/CCu > 6); while In complexes can be restricted to [(In3+)(OH-)(EDTA4-)] (pH 10-12; CEDTA/CIn = 2) and/or [(In3+)(ASP2-)₂] (pH 7-9; CASP/CIn = 5). These results indicate that the added amount of complex reagents should be determined by calculations and not the stoichiometric ratio. The reduction potential of homogenized metal complex is measured by cyclic voltammetry (CV) measurements and evaluated by Nernst’s equation using the overall stability constants. CuIn alloy nanoparticles with a small amount of byproduct (In nanoparticles) are successfully synthesized. The CI precursor films are spin-coated onto the substrate using a 2-propanol dispersion. Then the films are converted into CIS solar cells; which show a maximum conversion efficiency of 2.30%. The relationship between the open circuit potential; short circuit current density; and fill factor indicate that smoothing of the CIS films and improving the crystallinity and thickness increase the solar cell conversion efficiency.
Inverted device structure is a more stable configuration than regular device structure for solution-processed organic solar cells. However, most of the solution-processed small molecule organic solar cells (SM-OSCs) reported in literatures were using regular device structure, and regular device normally exhibits a higher efficiency than that of inverted device. Herein, a representative small molecule DR3TBDTT was selected to figure out the reason for photovoltaic performance differences between regular and inverted devices. The mechanisms for a reduced open-circuit voltage (Voc) and fill factor (FF) in the inverted device were studied. The reduced voltage is due to the vertical phase separation with excess PC71BM near air/blend surface, which leads to a reduction in build-in voltage and unbalance charge transport in inverted device. The unfavorable DR3TBDTT crystallite orientation along the blend film thickness is another reason the reduced FF, and DR3TBDTT forms a preferential edge-on orientation in the top layer of blend film and face-on crystallites in the bottom layer of blend film. This study illustrates that the morphology plays a key role on photovoltaic performance difference between regular and inverted structured devices, and provides useful guidelines for further optimizing the morphology of solution-processed SM-OSCs.
Planar perovskite solar cells (PSCs) have gained great interest due to their low-temperature solution preparation and simple process. In inverted planar PSCs, an additional buffer layer is usually needed on the top of the PCBM electron-transport layer (ETL) to enhance the device performance. In this work, we used a new buffer layer, zirconium acetate (Zr(Ac)4). The inclusion of the Zr(Ac)4buffer layer leads to the increase of FF from ∼68% to ∼79% and PCE from ∼14% to ∼17% in the planar PSCs. The UPS measurement indicates that the Zr(Ac)4layer has a low HOMO level of -8.2 eV, indicating that the buffer layer can act as a hole-blocking layer. Surface morphology and surface chemistry investigations reveal that the elements I, MA and Pb can diffuse across the PCBM ETL, damaging the device performance. The covering Zr(Ac)4molecules fill in the pinholes of the PCBM layer and effectively block the ions/molecules of the perovskite from diffusion across the ETL. The resulting more robust PCBM/Zr(Ac)4ETL leads to weaker ionic charge accumulation and lower diode leakage current. The double role of hole-and-ion blocking of the Zr(Ac)4layer explains the improved FF and PCE in the PSCs.
Polymer solar cells are a promising technology for the commercialization of low cost, large scale organic solar cells. With the evolution of high efficiency (>13%) non-fullerene polymer solar cells, the stability of the cells has become a crucial parameter to be considered. Among the several degradation mechanisms of polymer solar cells, burn-in photo-degradation is relatively less studied. Herein, we present the first systematic study of photo-degradation of novel PBDB-T:ITIC fullerene-free polymer solar cells. The thermally treated and as-prepared PBDB-T:ITIC solar cells were exposed to continuous 1 sun illumination for 5 hours. The aged devices exhibited rapid losses in the short-circuit current density and fill factor. The severe short-circuit current and fill factor burn in losses were attributed to trap mediated charge recombination, as evidenced by an increase in Urbach energy for aged devices.
Employing a layer of bulk-heterojunction (BHJ) organic semiconductors on top of perovskite to further extend its photoresponse is considered as a simple and promising way to enhance the efficiency of perovskite-based solar cells, instead of using tandem devices or near infrared (NIR)-absorbing Sn-containing perovskites. However, the progress made from this approach is quite limited because very few such hybrid solar cells can simultaneously show high short-circuit current (JSC ) and fill factor (FF). To find an appropriate NIR-absorbing BHJ is essential for highly efficient, organic, photovoltaics (OPV)/perovskite hybrid solar cells. The materials involved in the BHJ layer not only need to have broad photoresponse to increase JSC , but also possess suitable energy levels and high mobility to afford high VOC and FF. In this work, a new porphyrin is synthesized and blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to function as an efficient BHJ for OPV/perovskite hybrid solar cells. The extended photoresponse, well-matched energy levels, and high hole mobility from optimized BHJ morphology afford a very high power conversion efficiency (PCE) (19.02%) with high Voc , JSC , and FF achieved simultaneously. This is the highest value reported so far for such hybrid devices, which demonstrates the feasibility of further improving the efficiency of perovskite devices.
An all-small-molecule ternary solar cell is achieved with a power conversion efficiency of 10.48% by incorporating phenyl-C71 -butyric-acid-methyl ester (PC71 BM) into a nonfullerene binary system. The addition of PC71 BM is found to modulate the film morphology by improving the domain purity and decreasing the domain size. This modulation facilitates charge generation and suppresses charge recombination, as manifested by the significantly enhanced short-circuit current density and fill factor. The results correlate the domain characteristics with the device performance and offer new insight from the perspective of morphology modulation for constructing efficient ternary devices.