Semitransparent Selenium Solar Cells as a Top Cell for Tandem Photovoltaics

Abstract

Trigonal selenium (Se) is an elemental, direct‐bandgap (1.95 eV) semiconductor with a low processing temperature, which could be a suitable top absorber for tandem solar cell applications. For incorporation in tandem architectures, both sides of the Se cell should be semitransparent. However, all reported Se solar cells have metallic back contacts. To demonstrate the potential feasibility of Se as a wide‐bandgap absorber for tandems, herein, bifacial single‐junction selenium solar cells with device areas above 0.4 cm2 are reported. When illuminating through the n‐type contact, the bifacial cell power conversion efficiency (PCE) is 5.2%, similar to a standard monofacial cell. The efficiency is lower (2.7%) when illuminating through the p‐type contact, which is attributed to low carrier diffusion lengths and lifetimes in selenium. This suggests the necessity to invert the typical single‐junction device structure when incorporating it into a tandem device.

Selenium Thin-Film Solar Cells with Cadmium Sulfide as a Heterojunction Partner

“…In this work, the suitability of CdS as an n-type heterojunction partner to poly-Se is investigated by fabricating single-junction solar cells employing the champion structure fluorine-doped tin oxide (FTO)/n-type buffer/Te/Se/MoO x /Au, as reported by Todorov et al This device architecture is still in the superstrate configuration, but it serves as a starting point to examine the candidacy of CdS prior to inverting the structure. The process flow is based on an optimized in-house recipe, resulting in devices with a PCE = 5.2%, where RF-sputtered ZnMgO has been used as the n-type contact material . The energy band positions of CdS and poly-Se are investigated using photoelectron spectroscopy combined with optical transmission measurements, and the overall device performance is finally assessed of the first-reported CdS/Se solar cell.…”

“…In addition to its suitable bandgap, poly-Se is an inexpensive single-element semiconductor, it has a high absorption coefficient (α > 10 5 cm –1 ) in the visible region, and its low melting point ∼220 °C makes it process-compatible for low-cost monolithic integration with most bottom cell material candidates. However, recent work on bifacial selenium solar cells shows how the PCE is reduced by a factor of ∼2 when illuminating through the p-type contact . This reduction is believed to be a consequence of low carrier lifetimes and carrier diffusion lengths in poly-Se thin films.…”

“…However, recent work on bifacial selenium solar cells shows how the PCE is reduced by a factor of ∼2 when illuminating through the p-type contact. 12 This reduction is believed to be a consequence of low carrier lifetimes and carrier diffusion lengths in poly-Se thin films. As a result, charge carriers generated outside the depletion region would have a low probability of being collected before recombining.…”

“…The process flow is based on an optimized in-house recipe, resulting in devices with a PCE = 5.2%, where RFsputtered ZnMgO has been used as the n-type contact material. 12 The energy band positions of CdS and poly-Se are investigated using photoelectron spectroscopy combined with optical transmission measurements, and the overall device performance is finally assessed of the first-reported CdS/Se solar cell.…”

Solution Processed Fabrication of Se–Te Alloy Thin Films for Application in PV Devices

“…Although Se did not get much attention as individual photovoltaic material in past years, it has been part of some high-efficiency metal chalcogenide solar cell materials like Cu­(In,Ga)­Se 2 , Cu 2 ZnSnSe 4 , and Cd­(Se,Te) . Recently, few research groups have again started working on pure Se-based photovoltaic devices. − The reason for this can be their properties and ease of fabrication. Se has a high absorption coefficient and can be fabricated at temperatures below its melting point which is 220 °C, making it a material suitable for scalable deposition.…”