Aina Quintilla

Understanding the effect of solvent vapor annealing on solution-processed A–D–A oligothiophene bulk-heterojunction solar cells: the role of alkyl side chains

Solution-processed bulk heterojunction solar cells consisting of the previously developed dithienopyrrole containing A–D–A oligothiophenes (A = acceptor, D = donor unit) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) with power conversion efficiency up to 7.1% after solvent vapor annealing (SVA) are demonstrated. The influence of the position of the alkyl side chains attached to the thiophene units on the SVA, and the usage of either PC61BM or PC71BM as acceptor, is investigated in more detail by negative secondary ion mass spectrometry (SIMS), Kelvin probe force microscopy (KPFM), photoluminescence (PL), and grazing-incidence X-ray diffraction spectroscopy (GIWAXS). It was found that besides increased crystallinity and domain sizes, the active layers consisting of two different isomers which we will refer to in the following as isomer 1 or isomer 2 had different compositions after SVA treatment. In the former, a more or less homogeneously-mixed D:A blend was observed, whereas the latter showed a vertical gradient of PCBM in the active layer and much stronger phase segregation on the surface. These findings correlate well with the differences in solar cell performance of both isomers, before and after SVA.

Highly stable solution processed metal-halide perovskite lasers on nanoimprinted distributed feedback structures

We report on the performance and stability of distributed feedback lasers based on the solution-processed methylammonium lead iodide perovskite (CH3NH3PbI3). The CH3NH3PbI3 layers are processed via solution-casting in ambient atmosphere onto nanoimprinted second order Bragg gratings. This way, we achieve highly polarized surface-emitted lasing at room temperature with a linewidth of less than 0.2 nm and a laser threshold of 120 kW/cm2. The lasing is stable; no change in the laser emission within 15 h of pulsed excitation with a repetition rate of 1 kHz (corresponding to >5 × 107 pulses) is observed, exceeding the stability achieved for solution processed organic semiconductor lasers. Furthermore, adjustment of the grating period allowed the lasing wavelength to be varied over the entire bandwidth of the amplified spontaneous emission (between 781 and 794 nm). The fabrication process of nanoimprinting followed by solution-casting of the gain material demonstrates that stable CH3NH3PbI3 lasers are compatible with scalable production technologies and offers a route towards electrically pumped diode architectures.

Colloidally stable selenium@copper selenide core@shell nanoparticles as selenium source for manufacturing of copper-indium-selenide solar cells.

Selenium nanoparticles with diameters of 100-400nm are prepared via hydrazine-driven reduction of selenious acid. The as-prepared amorphous, red selenium (a-Se) particles were neither a stable phase nor were they colloidally stable. Due to phase transition to crystalline (trigonal), grey selenium (t-Se) at or even below room temperature, the particles merged rapidly and recrystallized as micronsized crystal needles. As a consequence, such Se particles were not suited for layer deposition and as a precursor to manufacture thin-film CIS (copper indium selenide/CuInSe2) solar cells. To overcome this restriction, Se@CuSe core@shell particles are presented here. For these Se@CuSe core@shell nanoparticles, the phase transition a-Se→t-Se is shifted to temperatures higher than 100°C. Moreover, a spherical shape of the particles is retained even after phase transition. Composition and structure of the Se@CuSe core@shell nanostructure are evidenced by electron microscopy (SEM/STEM), DLS, XRD, FT-IR and line-scan EDXS. As a conceptual study, the newly formed Se@CuSe core@shell nanostructures with CuSe acting as a protecting layer to increase the phase-transition temperature and to improve the colloidal stability were used as a selenium precursor for manufacturing of thin-film CIS solar cells and already lead to conversion efficiencies up to 3%.

Multipass inkjet printed planar methylammonium lead iodide perovskite solar cells

We report the fabrication and optimization of multipass inkjet-printed perovskite solar cells. The presented process allows excellent control of crystallization dynamics and thickness of the perovskite layer. In order to obtain a homogenous perovskite film of large and densely packed crystals on a planar TiO2 electron transport layer we make use of an additional vacuum annealing step. Its beneficial impact was characterized in terms of device performance and laser beam induced current mapping as well as atomic force microscopy. The optimized fabrication methods resulted in power conversion efficiencies up to 11.3% of multipass inkjet printed perovskite solar cells. The spin coated reference devices reached 12.8% power conversion efficiency. With these first results, we advance the field of digital printing techniques for perovskites and demonstrate a major step towards highly efficient, low-cost and less Pb-waste producing perovskite solar cells.

Cu(In,Ga)Se2 thin-film solar cells based on a simple sputtered alloy precursor and a low-cost selenization step

High-efficiency thin-film solar cells based on Cu(In,Ga)Se2 are often formed by depositing precursor films and using a subsequent selenization step. We demonstrate a simple and cost-efficient approach simplifying both process steps by using a ternary Cu-In-Ga alloy targetforsputterdepositionoftheprecursorlayerandbyusingasimplenonvacuum selenization reactionbasedonelementalselenium.Inthiscontributionweexamineindetailthecharacteristics of the precursor layers. The sputter growth is governed by a segregation of In-rich islands on top of a closed Cu-rich base. With optimized layers we could achieve conversion efficiencies well above 13% without the use of antireflective coating or metallic grids. The influence of the selenization duration on morphology and performance is discussed. C 2011 Society of Photo-Optical

Infiltrated photonic crystals for light-trapping in CuInSe 2 nanocrystal-based solar cells

Solution processable nanocrystal solar cells combine the advantages of low-cost printing and wide range of accessible absorber materials, however high trap densities limit performance and layer thickness. In this work we develop a versatile route to realize the infiltration of a photonic crystal, with copper indium diselenide nanocrystal ink. The photonic crystal allows to couple incident light into pseudo-guided modes and thereby enhanced light absorption. For the presented design, we are able to identify individual guided modes, explain the underlying physics, and obtain a perfect match between the measured and simulated absorption peaks. For our relatively low refractive index layers, a 7% maximum integrated absorption enhancement is demonstrated.

Temperature Variation-Induced Performance Decline of Perovskite Solar Cells

This paper reports on the impact of outdoor temperature variations on the performance of organo metal halide perovskite solar cells (PSCs). It shows that the open-circuit voltage ( VOC) of a PSC decreases linearly with increasing temperature. Interestingly, in contrast to these expected trends, the current density ( JSC) of PSCs is found to decline strongly below 20% of the initial value upon cycling the temperatures from 10 to 60 °C and back. This decline in the current density is driven by an increasing series resistance and is caused by the fast temperature variations as it is not apparent for solar cells exposed to constant temperatures of the same range. The effect is fully reversible when the devices are kept illuminated at an open circuit for several hours. Given these observations, an explanation that ascribes the temperature variation-induced performance decline to ion accumulation at the contacts of the solar cell because of temperature variation-induced changes of the built-in field of the PSC is proposed. The effect might be a major obstacle for perovskite photovoltaics because the devices exposed to real outdoor temperature profiles over 4 h showed a performance decline of >15% when operated at a maximum power point.

Time-resolved spectroscopy of charge transfer phenomena in organic solar cells

Geminate recombination of photo-generated excitons represents a considerable loss mechanism in polymer solar cells. We apply time-resolved photoluminescence (TRPL) to study the radiative recombination which accompanies the process of charge generation. A streak camera is used, which is sensitive for both the photoluminescence (PL) from the initially excited singlet excitons and the weaker emission from charge transfer (CT) states. The latter are formed at internal interfaces when the polymer is blended with a fullerene acceptor. We draw a comparison between our results for two polymers, P3HT and PTB7, respectively, which were studied in blends with the fullerene derivative PCBM. In addition, pristine films were investigated, allowing for the identification of interfacial features in the blends. For both polymers, the PL of the singlet states was rapidly quenched in blends with PCBM. In P3HT, time constants of about 40 ps were recorded for the singlet exciton decay and related to exciton diffusion, whereas the PL of PTB7 was almost completely quenched within the first 3 ps. The decay rates of the emissive CT excitons were 2-3 orders of magnitude smaller than those of the singlet state. Yet, due to their slower dynamics (~ 500 ps), they could be separated from the superimposed singlet emission. The CT decay times in blends with P3HT exhibited no significant temperature dependence, indicating that thermally driven dissociation of emissive excitons is unlikely. For blends with PTB7, however, a faster decay of the CT emission was obtained at room temperature.

Scalable and low cost fabrication methods for wavelength tunable solution processed perovskite distributed feedback lasers

There is a growing interest in the use of metal halide perovskites with different compositions as highly efficient light emitters among the whole visible spectral region [1]. Several groups have observed amplified spontaneous (ASE) emission in thin perovskite films. The gain and threshold values for ASE are very promising, motivating the current effort to develop practical devices that lase.

Multipass inkjet printing of methylammonium lead iodide for planar perovskite solar cells(Conference Presentation)

We show inkjet printed state-of-the-art perovskite solar cells with efficiencies of up to 12% which is an important step towards fully printed large scale production of photovoltaic perovskite devices. In comparison, the spin-coated reference achieves 13% efficiency. In both cases, the solar cell absorbers are prepared using a one-step process on a TiO2 compact layer without mesoporous intermediate layer as electron transport material and spiroMeOTAD as hole transport material. Moreover, we show that controlling printing parameters, like drop spacing and size, is essential to optimizing the final perovskite performance. Whereas parameters were initially controlled to be consistent with a final layer thicknesses known from literature, subsequent processes were aimed at also controlling crystallinity and roughness. To demonstrate the homogeneity of the printed devices, light beam induced current measurements (LBIC) were made. To evaluate the quality of the perovskite layer and the charge transfer efficiency in the device, time resolved photoluminescence measurements were conducted on the perovskite with and without electrical transport layers. Light soaking effects were also investigated and evaluated. Important differences between printed and spin-coated devices will be outlined, as well as other relevant parameters to optimize printed device performance.