CTS and CZTS for solar cells made by pulsed laser deposition and pulsed electron deposition

Research output: Book/ReportPh.D. thesis

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Abstract

This thesis concerns the deposition of thin films for solar cells using pulsed laser deposition (PLD) and pulsed electron deposition (PED). The aim was to deposit copper tin sulfide (CTS) and zinc sulfide (ZnS) by pulsed laser deposition to learn about these materials in relation to copper zinc tin sulfide (CZTS), a new material for solar cells. We were the first research group to deposit CTS by pulsed laser deposition and since this is a potential solar cell material in its own right we experimented with CTS solar cells in parallel with CZTS. Both CTS and CZTS contain only Earth-abundant elements, which make them promising alternatives to the commercially successful solar cell material copper indium gallium diselenide (CIGS). Complementing our group's work on pulsed laser deposition of CZTS, we collaborated with IMEM-CNR in Parma, Italy, to deposit CZTS by pulsed electron deposition for the first time. We compared the results of CZTS deposition by PLD at DTU in Denmark to CZTS made by PED at IMEM-CNR, where CIGS solar cells have successfully been fabricated at very low processing temperatures. The main results of this work were as follows: Monoclinic-phase CTS films were made by pulsed laser deposition followed by high temperature annealing. The films were used to understand the double band gap that we and other groups observed in the material. The Cu-content of the CTS films varied depending on the laser fluence (the laser energy per pulse and per area). The material transfer from the multicomponent target to the film was generally not stoichiometric. The annealed CTS films could not be more than about 700 nm thick to avoid exfoliation and bubbles in the films. The CTS solar cells have therefore not yet been optimized and the maximum efficiency of our CTS solar cells was 0.3 % so far. The aim of using pulsed electron deposition was to make CZTS at a low processing temperature, avoiding the 570 °C annealing step used for our pulsed laser deposited solar cells. Preliminary solar cells had an efficiency of 0.2 % with a 300 °C deposition step without annealing. Further process control is needed. With both pulsed laser deposition and pulsed electron deposition we found that the Cu content of the films could be altered by changing the fluence (in PLD) or the voltage and pressure (in PED). SnS evaporated preferentially from the multicomponent target at low laser intensity and low pulsed electron beam voltage. Finally we compared two different lasers for deposition of CZTS and CTS: a 248 nm, 20 ns KrF excimer laser and a 355 nm, 6 ns Nd:YAG laser. While my colleague found that CZTS was best deposited with the 248 nm laser which has a high enough photon energy to exceed the band gap of the ZnS phase in the target, I found that it did not make a large difference which of the two lasers was used for the deposition of CTS. Due to the longer pulses leading to a lower laser intensity at a given fluence, the 248 nm laser afforded a somewhat wider fluence range for optimal Cu-content in the films. Droplets of up to micron size were found on the films of CZTS and CTS by both pulsed laser deposition and pulsed electron deposition. The number of droplets diminished when the fluence was reduced in PLD or when the accelerating voltage was reduced in PED. The change in laser wavelength from 355 nm to 248 nm in contrast had no impact on the number of droplets on the CTS films at a given fluence.
Original languageEnglish
PublisherDTU Fotonik
Number of pages269
Publication statusPublished - 2016

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