Robert Gehlhaar

Pinhole-free perovskite films for efficient solar modules

We report on a perovskite solar module with an aperture area of 4 cm2 and geometrical fill factor of 91%. The module exhibits an aperture area power conversion efficiency (PCE) of 13.6% from a current–voltage scan and 12.6% after 5 min of maximum power point tracking. High PCE originates in pinhole-free perovskite films made with a precursor combination of Pb(CH3CO2)2·3H2O, PbCl2, and CH3NH3I.

An electron beam evaporated TiO2 layer for high efficiency planar perovskite solar cells on flexible polyethylene terephthalate substrates

The TiO2 layer made by electron beam (e-beam) induced evaporation is demonstrated as an electron transport layer (ETL) in high efficiency planar junction perovskite solar cells. The temperature of the substrate and the thickness of the TiO2 layer can be easily controlled with this e-beam induced evaporation method, which enables the usage of different types of substrates. Here, perovskite solar cells based on CH3NH3PbI3−xClx achieve power conversion efficiencies of 14.6% on glass and 13.5% on flexible plastic substrates. The relationship between the TiO2 layer thickness and the perovskite morphology is studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Our results indicate that the pinholes in the thin TiO2 layer lead to pinholes in the perovskite layer. By optimizing the TiO2 thickness, perovskite layers with substantially increased surface coverage and reduced pinhole areas are fabricated, increasing overall device performance.

Rapid composition screening for perovskite photovoltaics via concurrently pumped ultrasonic spray coating

Transitioning perovskite photovoltaics from the rapid progress in lab-scale devices to industrially viable large area modules is a key challenge for the economic breakthrough of the technology. In this work, we demonstrate ultrasonic spray coating as a scalable and versatile linear deposition technique for high efficiency perovskite photovoltaics. We show the versatility of concurrently pumped ultrasonic spray coating by rapidly and precisely optimizing precursor ratios based on PbCl2, Pb(CH3CO2)2·3H2O, PbBr2, CH3NH3Br, and CH3NH3I to achieve highly crystalline and pinhole-free layers. Initial power conversion efficiencies of 15.7% for small scale devices and 11.7% for 3.8 cm2 modules were achieved with current–voltage sweeps and tracked to 13.4% for devices and 10.4% for modules under continuous illumination and bias at the maximum power point. Process versatility is further demonstrated with the in situ bandgap control in CH3NH3PbIXBr3−X layers.

Enhancement of the photovoltaic performance in P3HT: PbS hybrid solar cells using small size PbS quantum dots

Different approaches of surface modification of the quantum dots (QDs), namely, solution-phase (octylamine, octanethiol) and post-deposition (acetic acid, 1,4-benzenedithiol) ligand exchange were used in the fabrication of hybrid bulk heterojunction solar cell containing poly (3-hexylthiophene) (P3HT) and small (2.4u2009nm) PbS QDs. We show that replacing oleic acid by shorter chain ligands improves the figures of merit of the solar cells. This can possibly be attributed to a combination of a reduced thickness of the barrier for electron transfer and an optimized phase separation. The best results were obtained for post-deposition ligand exchange by 1,4-benzenedithiol, which improves the power conversion efficiency of solar cells based on a bulk heterojunction of lead sulfide (PbS) QDs and P3HT up to two orders of magnitude over previously reported hybrid cells based on a bulk heterojunction of P3HT:PbS QDs, where the QDs are capped by acetic acid ligands. The optimal performance was obtained for solar cells with 69u2009wt.u2009% PbS QDs. Besides the ligand effects, the improvement was attributed to the formation of an energetically favorable bulk heterojunction with P3HT, when small size (2.4u2009nm) PbS QDs were used. Dark current density-voltage (J-V) measurements carried out on the device provided insight into the working mechanism: the comparison between the dark J-V characteristics of the bench mark system P3HT:PCBM and the P3HT:PbS blends allows us to conclude that a larger leakage current and a more efficient recombination are the major factors responsible for the larger losses in the hybrid system.

Interconnection Optimization for Highly Efficient Perovskite Modules

This paper reports on the analysis and comparison of mechanical and laser patterning in the fabrication of perovskite thin-film photovoltaic modules. Besides stability, device upscaling and module fabrication is akey challenge for the commercialization of perovskite photovoltaics. Here, the focus is on the optimization of the P2 interconnection that represents the electrical connection between serially connected cells in a module. The specific contact resistivity for P2 interconnection is determined by using an enhanced transmission line method. Mechanical or laser patterning are used to fabricate 4 cm2 modules with aperture area efficiencies of up to 15.3% and geometrical fill factors as high as 94%. With the application of a simulation program with an integrated circuit emphasis-based electrical device model, the interconnection losses are quantified, and optimal designs for perovskite modules are presented.

Highly efficient perovskite solar cells with crosslinked PCBM interlayers

Commercially available phenyl-C61-butyric acid methyl ester (PCBM) is crosslinked with 1,6-diazidohexane (DAZH), resulting in films resistant to common solvents used in perovskite solar cell processing. By using crosslinked PCBM as an interlayer and (HC(NH2)2)0.66(CH3NH3)0.34PbI2.85Br0.15 as the active layer, we achieve small area devices and modules with a maximum steady-state power conversion efficiency of 18.1% and 14.9%, respectively.

Charge transport and recombination in P3HT:PbS solar cells

The charge carrier transport in thin film hybrid solar cells is analyzed and correlated with device performance and the mechanisms responsible for recombination loss. The hybrid bulk heterojunction consisted of a blend of poly(3-hexylthiophene) (P3HT) and small size (2.4 nm) PbS quantum dots (QDs). The charge transport in the P3HT:PbS blends was determined by measuring the space-charge limited current in hole-only and electron-only devices. When the loading of PbS QDs exceeds the percolation threshold, a significant increase of the electron mobility is observed in the blend with PbS QDs. The hole mobility, on the other hand, only slightly decreased upon increasing the loading of PbS QDs. We also showed that the photocurrent is limited by the low shunt resistance rather than by space-charge effects. The significant reduction of the fill factor at high light intensity suggests that under these conditions the non-geminate recombination dominates. However, at open-circuit conditions, the trap-assisted recombination dominates over non-geminate recombination.

Scalable perovskite/CIGS thin-film solar module with power conversion efficiency of 17.8%

All-thin film perovskite/CIGS multijunction solar modules, combining a semi-transparent perovskite top solar module stacked on a CIGS bottom solar module, are a promising route to surpass the efficiency limits of single-junction thin-film solar modules. In this work, we present a scalable thin-film perovskite/CIGS photovoltaic module with an area of 3.76 cm2 and a power conversion efficiency of 17.8%. Our prototype outperforms both the record single-junction perovskite solar module of the same area as well as the reference CIGS solar module. The presented perovskite/CIGS thin-film multijunction solar module makes use of the “4-terminal architecture”, which stacks the perovskite solar module in superstrate configuration on top of the CIGS solar module in substrate configuration. Both submodules apply a scalable interconnection scheme that can accommodate scale-up towards square meter scale thin-film multijunction solar modules. In order to identify the future potential of the presented stacked perovskite/CIGS thin-film solar module, we quantify the various losses in the presented prototype and identify the key challenges of this technology towards very high power conversion efficiencies.

The Influence of Alkoxy Substitutions on the Properties of Diketopyrrolopyrrole-Phenyl Copolymers for Solar Cells

A previously reported diketopyrrolopyrrole (DPP)-phenyl copolymer is modified by adding methoxy or octyloxy side chains on the phenyl spacer. The influence of these alkoxy substitutions on the physical, opto-electronic properties, and photovoltaic performance were investigated. It was found that the altered physical properties correlated with an increase in chain flexibility. Well-defined oligomers were synthesized to verify the observed structure-property relationship. Surprisingly, methoxy substitution on the benzene spacer resulted in higher melting and crystallization temperatures in the synthesized oligomers. This trend is not observed in the polymers, where the improved interactions are most likely counteracted by the larger conformational possibilities in the polymer chain upon alkoxy substitution. The best photovoltaic performance was obtained for the parent polymer: fullerene blends whereas the modifications on the other two polymers result in reduced open-circuit voltage and varying current densities under similar processing conditions. The current densities could be related to different polymer: fullerene blend morphologies. These results show that supposed small structural alterations such as methoxy substitution already significantly altered the physical properties of the parent polymer and also that oligomers and polymers respond divergent to structural alterations made on a parent structure.

Four-terminal organic solar cell modules with increased annual energy yield

The authors present experimental results on mechanically stacked organic solar modules and their advantage over standard tandem architectures. A four-terminal configuration of two single junction modules with complementary absorbing active layers uses the more efficient energy conversion of a tandem structure without the necessity of matching currents or voltages of electrically connected subcells. The presented combination of semitransparent and opaque solar cells consists of solution processed polymer-fullerene blends as active materials. A cost-effective mechanical scribing process is applied for the patterning of the deposited layers. The best devices have an efficiency of over 6.5% on an aperture area of 16 cm2 which equals a gain of 30% over the best single junction module fabricated by the same process. Optical simulations demonstrate a 32% increased annual energy output of a mechanically stacked device in comparison to a monolithic tandem structure using an equivalent geometry.