Search

We investigate how post-deposition laser annealing can be used to improve structural and electronic quality of room-temperature deposited CdTe. We use continuous-wave, 1064 nm laser light to anneal CdTe solar cell stacks prior to back contact deposition. Sub-bandgap optical absorption measurements by photothermal deflection spectroscopy show a reduction of sub-bandgap defects due to the annealing process. Since the 1064 nm light is only partially absorbed, in situ monitoring of the transmitted light during laser annealing gives real-time information about changes in the material. These results reveal an evolution of electronic defect annealing and surface roughness modification with laser exposure time. This hypothesis is supported by electron microscopy. Two distinct annealing regimes emerge: one at low laser power where electronic defect annealing saturates after about one minute exposure and another at high power where structural defects are annealed after several minutes exposure. Temperatures reached during laser annealing are estimated by finite element modeling of the thermal transport due to heat generation from optical absorption.

Presented here are the results of a three dimensional, finite element simulation that models pulsed, ultraviolet (UV) laser annealing of polycrystalline CdTe. The model considers heat generated by the absorption of a 25 ns, 248 nm laser pulse normally incident to a 5 μm thick CdTe thin film deposited on a polycrystalline alumina substrate. In particular, focus is on the spatial and temporal distribution of temperature from laser fluences that achieve a sub-melting condition. The model shows that there are very large temperature gradients both in depth and in-plane directions. These predictions, as well as the onset of melting, are confirmed with cross sectional scanning electron microscopy. Additionally, the model predicts that the heat generated dissipates rapidly after the pulse has ended. This has implications if pulse trains are to be used experimentally.

For the production of high efficiency thin film, Cu(In,Ga)Se2 solar cells, absorber layers with grain sizes of a few hundred nanometers and without detrimental secondary phases are favored. Co-electrodeposition offers a low-cost and material efficient synthesis route, where, in a single step, films containing CuInSe2 are formed. However, the material is nanocrystalline, constitutes multiple phases and has poor photovoltaic properties 1. Therefore a subsequent annealing step is required to produce absorber layers suitable for use in photovoltaic devices. Laser annealing has been demonstrated to improve crystallinity, stimulate atomic diffusion and develop opto-electronic properties when compared to the precursor 2. In this work, high resolution X-ray diffraction was used in order to assess the presence of secondary phases in the absorber layer. All diffractograms of laser annealed films exhibited an additional, unknown peak, measurable through the full depth of the material which is independent of precursor composition, annealing time or laser flux. Evaluation of literature on codeposited CuInSe2, combined with Rietveld refinement suggests a number of possible identities for this peak. The candidates in order of most likely to least likely are structural defects, In2Se3, and CuIn3Se5. We consider the impact that each of these would have on a device formed via this process and thus its success as a new manufacturing route for CuInSe2 solar cells.

Cu 2 ZnSnS 4 (CZTS) may exhibit both kesterite and stannite polymorphs and shows promise as an absorber layer in thin film photovoltaic solar cells to be produced at terawatt scales. This study examines the effects of CZTS polymorphism and inhomogeneous distributions of CZTS polymorphs on device characteristics under scenarios of single phase films, a sinusoidal variation between kesterite and stannite with depth, and single phase films with thin layers of the other polymorph at both interfaces. In general, stannite-only devices are predicted to have higher efficiency than kesterite-only devices and sinusoidal grading results in efficiency between those of the pure phases. However, the device performance is relatively insensitive to the wavelength of the sinusoidal grading and rather is very sensitive to the phase present at the CdS interface. Predicted AM1.5 current-voltage (J-V) curves and descriptive metrics as well as wavelength-resolved quantum efficiencies are reported for all models. Based on these results, we propose control of cation ordering in CZTSSe as a mechanism for device design using bandgap grading and interface engineering without variation of stoichiometry.

In order to upscale the production of thin film solar cells a cost effective and simple synthesis technique is required. Keeping this in mind we have investigated the effect of electrochemical deposition (ED) and inherently low thermal budget rapid thermal annealing (RTA) processing of CuInSe 2 in sulfur atmosphere. X-ray diffraction ( -2 ) scans indicate increased grain size and improved crystallinity after RTA of ED films. Scanning electron microscopy images (SEM) suggest changes in surface morphol ogy after sulfur incorporation. Raman spectroscopy results and temperature dependent conductivity measurements are also discussed in the paper. CuIn(S,Se) 2 , electrochemical deposition and rapid thermal annealing 1. INTRODUCTION Thin film solar cell is a fast growing sector of solar cell industry with its major advantages of potential cost reduction and scalability. These advantages over the present high throughput crystalline Si technology will bring the cost per watt of solar energy at grid parity. Of the relatively less defect sensitive photovoltaic (PV) materials for thin film solar cells, materials with chalcogenide and kesterite structure are considered strong contenders. CuInSe

Cu(In,Ga)Se2 (CIGS) thin film photovoltaic absorber layers are primarily synthesized by vacuum based techniques at industrial scale. In this work, we investigate non-vacuum film synthesis by electrochemical deposition coupled with pulsed laser annealing (PLA) and or continuous wave laser annealing (CWLA) using 1064 nm laser. PLA results indicate that at high fluence (≥100 mJ/cm2) CuInSe2 films melt and dewet on both Mo and Cu substrates. In the submelt PLA regime (≤70 mJ/cm2) no change in XRD results is recorded. However CWLA at 50 W/cm2 for up to 45 s does not result in melting or dewetting of the film. XRD and Raman data indicate more than 80% reduction in full width at half maximum (FWHM) in their respective main peaks for annealing time of 15 s or more. No other secondary phases are observed in XRD or Raman spectrum. These results might help us in setting up the foundation for processing CIGS through an entirely non-vacuum process.