Beyond 6 % Efficient Ultrasonic Spray Deposited Sb₂S₃ Solar Cells from 100 nm Thick Pristine Absorbers for Semi-transparent Applications

Antimony sulphide (Sb₂S₃) is an emerging semiconductor material with excellent promise for future photovoltaic (PV) technologies owing to its superior optoelectronic properties coupled with earth-abundant and environmentally friendly elements. The relatively wide bandgap of Sb₂S₃ (1.7 eV) makes it wiser choice for application in semi-transparent PV and tandem solar cells. Although the material has potential even for single junction cells with a Shockley-Queisser efficiency limit of ≈ 28 % but the open circuit voltage deficiencies arising from defects and/or self-trapping of photogenerated carriers by lattice deformation limit the maximum efficiency to ≈ 16 %. Therefore, despite numerous attempts and various device architectures, the efficiency of Sb₂S₃ solar cells is still considerably low with an achieved maximum of 8 %, even for an absorber thickness ranging from 200 nm to over 2 µm. And, most of the high efficiency devices (7-8 %), which utilize an absorber thickness of at least 200 nm, are fabricated by either spin coating, chemical bath deposition (CBD) or vacuum deposition techniques. While CBD has the promise for large area upscaling, its throughput is relatively low and requires longer deposition time (≈ 4 hours). In this context, the study herein demonstrates 6.2 % efficient Sb₂S₃ solar cells from an absorber thickness of ≈ 100 nm deposited using industrially benign and scalable ultrasonic spray pyrolysis (USP) with high throughput (deposition time ≈ 20 min). The solar cells are apt for semi-transparent applications with an average visible transmittance of 27 % (400-800 nm) for the device stack with P3HT as hole transport material without the back metal contact. To the best of authors’ knowledge, the efficiency of 6.2 % is a reasonable record achieved for pristine 100 nm Sb₂S₃ absorbers devoid of any doping or post deposition treatment. The individual layers and solar cells are comprehensively characterized using advanced characterization techniques like photoelectron emission spectroscopy, photoluminescence, TRPL, and DLTS to understand the band energetics, device physics, defects and interfaces. The USP process parameters towards obtaining high quality absorbers leading to record efficiencies will be discussed in detail while emphasizing the role of defects on the performance of Sb₂S₃ solar cells. In addition, the roadmap towards complete semi-transparent Sb₂S₃ solar cells by replacing back metal contact, which is being pursued, will be presented.