Accelerating research on novel photovoltaic materials

The development of new materials typically takes many years or even decades. This has been particularly true for photovoltaic PV technologies, which require control of defects on the parts per million level and consist of relatively complex device structures comprising many elements and interfaces between materials. This means that optical and electronic properties can be difficult to pin down, and also heavily depend on the details of processing. Although processing often varies from lab to lab, complete protocols are rarely reported or accessible. It is suggested that the development of novel photovoltaic materials could be greatly stimulated if information and data is more openly shared, and FAIR data management is implemented in the research community. Massive storage of research results with rich metadata in an FAIR compliant open access database is envisioned as a great potential for acceleration in emerging PV materials developmen

The Effect of Cu Zn Disorder on Charge Carrier Mobility and Lifetime in Cu2ZnSnSe4

Cu Zn disorder is one possible origin for the limited efficiencies of kesterite solar cells and its impact on the band gap and band tails have been intensively studied. However, the effect on charge transport and recombination, which are key properties for solar cells, has not been investigated so far. Therefore, we probe the impact of the Cu Zn order on charge carrier mobility and lifetime. To this end, we change the Cu Zn order of a co evaporated Cu2ZnSnSe4 thin film by sequential annealing and probe the impact by time resolved terahertz spectroscopy. Aside from of the well known band gap shift, we find no significant change in mobility and lifetime with Cu Zn order. This finding indicates that Cu Zn disorder is not limiting efficiencies of kesterite solar cells at their current status by means of charge carrier recombination and transpor

Vertical integration of ultrafast semiconductor lasers

Lasers generating short pulses - referred to as ultrafast lasers - enable many applications in science and technology. Numerous laboratory experiments have confirmed that ultrafast lasers can significantly increase telecommunication data rates [1], improve computer interconnects, and optically clock microprocessors [2, 3]. New applications in metrology [4], supercontinuum generation [5], and life sciences with two-photon microscopy [6] only work with ultrashort pulses but have relied on bulky and complex ultrafast solid-state lasers. Semiconductor lasers are ideally suited for mass production and widespread applications, because they are based on a wafer-scale technology with a high level of integration. Not surprisingly, the first lasers entering virtually every household were semiconductor lasers in compact disk players. Here we introduce a new concept and make the first feasibility demonstration of a new class of ultrafast semiconductor lasers which are power scalable, support both optical and electrical pumping and allow for wafer-scale fabrication. The laser beam propagates vertically (perpendicularly) through the epitaxial layer structure which has both gain and absorber layers integrated. In contrast to edge-emitters, these lasers have semiconductor layers that can be optimized separately by using different growth parameters and with no regrowth. This is especially important to integrate the gain and absorber layers, which require different quantum confinement. A saturable absorber is required for pulse generation and we optimized its parameters with a single self-assembled InAs quantum dot layer at low growth temperatures. We refer to this class of devices as modelocked integrated external-cavity surface emitting lasers (MIXSEL). Vertical integration supports a diffraction-limited circular output beam, transform-limited pulses, lower timing jitter, and synchronization to an external electronic clock. The pulse repetition rate scales from 1-GHz to 100-GHz by simply changing the laser cavity length. This result holds promise for semiconductor-based high-volume wafer-scale fabrication of compact, ultrafast laser

Local growth of CuInSe2 micro solar cells for concentrator application

A procedure to fabricate CuInSe2 CISe micro absorbers and solar cells for concentrator applications is presented. The micro absorbers are developed from indium precursor islands, which are deposited on a molybdenum coated glass substrate back contact , followed by deposition of copper on top and subsequent selenization as well as selective etching of copper selenides. In order to compare the properties of the locally grown absorbers to those of conventional large area CISe films, we systematically examine the compositional and morphological homogeneity of the micro absorbers and carry out photoluminescence measurements. Preliminary devices for micro concentrator solar cell applications are fabricated by optimizing the copper to indium ratio and the size of the indium precursor islands. The resulting micro solar cells provide a characteristic I V curve under standard illumination conditions 1 su

Electron-beam-induced current at absorber back surfaces of Cu (In,Ga) Se2 thin-film solar cells

The following article appeared in Journal of Applied Physics 115.1 (2014): 014504 and may be found at http://scitation.aip.org/content/aip/journal/jap/115/1/10.1063/1.4858393The present work reports on investigations of the influence of the microstructure on electronic properties of Cu(In,Ga)Se2 (CIGSe) thin-film solar cells. For this purpose, ZnO/CdS/CIGSe stacks of these solar cells were lifted off the Mo-coated glass substrates. The exposed CIGSe backsides of these stacks were investigated by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements as well as by electron backscattered diffraction (EBSD). EBIC and CL profiles across grain boundaries (GBs), which were identified by EBSD, do not show any significant changes at Σ3 GBs. Across non-Σ3 GBs, on the other hand, the CL signals exhibit local minima with varying peak values, while by means of EBIC, decreased and also increased short-circuit current values are measured. Overall, EBIC and CL signals change across non-Σ3 GBs always differently. This complex situation was found in various CIGSe thin films with different [Ga]/([In]+[Ga]) and [Cu]/([In]+[Ga]) ratios. A part of the EBIC profiles exhibiting reduced signals across non-Σ3 GBs can be approximated by a simple model based on diffusion of generated charge carriers to the GBs.This work was supported in part by the BMU projects comCIGS and comCIGSII. R.C. acknowledges financial support from Spanish MINECO within the program Ramon y Cajal (RYC-2011-08521)

Metal acetate based synthesis of small sized Cu2ZnSnS4 nanocrystals effect of injection temperature and synthesis time

We report the colloidal synthesis of small sized Cu2ZnSnS4 CZTS nanocrystals NCs via a hot injection method using zinc and tin acetates in combination with copper acetylacetonate as metal precursors. A systematic investigation of the in amp; 64258;uence of the injection temperature in the range from 190 o C to 300 o C on the size distribution, composition and phase purity of CZTS nanocrystals has been performed. It has been found that temperature plays the key role in changing of the metal sources reactivity and in amp; 64258;uences the amp; 64257;nal composition of the nanocrystals. The mechanism of nanocrystal formation has been investigated by Raman spectroscopy of aliquots of their solutions. It starts from the formation of a Cu2 xS phase as a core followed by the incorporation of Zn2 and Sn4 atoms into its structure regardless of injection temperature

Cu Zn disorder in stoichiometric Cu2ZnSn S1 xSex 4 semiconductors A complementary neutron and anomalous X ray diffraction study

The quaternary compound semiconductor Cu2ZnSn S1 xSex 4 CZTSSe which crystallizes in the kesterite type structure, is a promising material to be used as p type absorber layer in thin film solar cell applications based on earth abundant elements. The absorber band tailing caused by an exceptionally high Cu Zn disorder is believed to be one of the reasons for the limited open circuit voltage in CZTSSe based photovoltaic devices. This work is an experimental study of the Cu Zn disorder in a unique set of single phase, stoichiometric CZTSSe mixed crystals, synthesized by solid state reaction, by means of neutron powder diffraction and anomalous X ray powder diffraction. The existence of Cu Zn disorder was revealed as the only intrinsic point defect in these mixed crystals within the detection limits of our measurements. The order parameter Q was calculated on the basis of the occurring CuZn and ZnCu anti site defects causing the Cu Zn disorder. Variations of the order parameter with anion composition and the effect on the optoelectronic properties of the partial substitution of Se with S in Cu2ZnSn S1 xSex 4 was elaborate

Minority and Majority Charge Carrier Mobility in Cu2ZnSnSe4 revealed by Terahertz Spectroscopy

The mobilities of electrons and holes determine the applicability of any semiconductor, but their individual measurement remains a major challenge. Here, we show that time resolved terahertz spectroscopy TRTS can distinguish the mobilities of minority and majority charge carriers independently of the doping type and without electrical contacts. To this end, we combine the well established determination of the sum of electron and hole mobilities from photo induced THz absorption spectra with mobility dependent ambipolar modeling of TRTS transients. The method is demonstrated on a polycrystalline Cu2ZnSnSe4 thin film and reveals a minority electron mobility of 128 cm2 V s and a majority hole carrier mobility of 7 cm2 V s in the vertical transport direction relevant for light emitting, photovoltaic and solar water splitting devices. Additionally, the TRTS analysis yields an effective bulk carrier lifetime of 4.4 ns, a surface recombination velocity of 6 104 cm s and a doping concentration of ca. 1016 cm 3, thus offering the potential for contactless screen novel optoelectronic material

Optical in situ monitoring during the synthesis of halide perovskite solar cells reveals formation kinetics and evolution of optoelectronic properties

The formation mechanism and the evolution of optoelectronic properties during annealing of chlorine derived methylammonium lead iodide MAPbI3 amp; 8722;xClx are investigated in detail combining in situ and ex situ optical and structural characterization. Using in situ optical reflectometry we are able to monitor the evolution of the MAPbI3 amp; 8722;xClx phase as a function of time and processing temperature. The formation kinetics is fitted using an improved Johnson Mehl Avrami Kolmogorov model and a delayed formation of MAPbI3 amp; 8722;xClx is found when chlorine is present in the precursor. This is verified by X ray diffraction and X ray fluorescence measurements. From absolute photoluminescence measurements we determine the implied Voc during film formation, which exhibits a maximum at a specific time during the annealing process. In conjunction with ex situ time resolved photoluminescence we deduce a decrease in the net doping density for increased annealing times, while the minority carrier lifetime stays constant. We thus demonstrate the potential of in situ optical spectroscopy to monitor and tailor the electronic properties of hybrid perovskites directly during film growth, which can be easily applied to different growth recipes and synthesis environment