Radiative Efficiency and Charge‐Carrier Lifetimes and Diffusion Length in Polycrystalline CdSeTe Heterostructures

Abstract

CdTe‐based photovoltaics is a rapidly developing energy technology with one of the lowest levelized costs of electricity. But nonradiative Shockley–Read–Hall (SRH) recombination limits the efficiency of CdTe solar cells. Partial mitigation of bulk and grain‐boundary SRH recombination was achieved by alloying CdTe with Se, and CdSexTe1−x absorbers are used in high‐efficiency solar cells. Recently, interface recombination was significantly reduced with alumina passivation, but other properties of Al2O3/CdSexTe1−x/Al2O3 heterostructures have not yet been investigated. Herein, further progress in understanding and developing polycrystalline heterostructures with x = 0.2 is reported. It is shown that external photoluminescence quantum yield is increased to 0.2%, quasi‐Fermi‐level splitting to 950 mV, minority carrier lifetimes to 750 ns, and mobility to 100 cm2 Vs−1. Such polycrystalline CdSexTe1−x electronic characteristics match or exceed CdTe single‐crystal properties. The resulting charge‐carrier diffusion length of ≈14 μm is several times greater than the absorber thickness in test structures and in typical CdTe solar cells. Herein, with passivation and absorbers, polycrystalline CdSeTe solar cell open‐circuit voltage is increased from the current 76% of the Shockley–Queisser limit to 82%, or close to 1 V is reported.

Identification of Recombination Losses in CdSe/CdTe Solar Cells from Spectroscopic and Microscopic Time‐Resolved Photoluminescence

“…The external radiative efficiency (ERE) is obtained using spectral integration [ 24 ] and is found to be (0.05 ± 0.01)% for Al 2 O 3 passivated sample and (0.03 ± 0.01)% for sample without passivation. As described in the next section, with Al 2 O 3 carrier lifetimes increase proportionally to ERE.…”

“…ERE was determined using integrating sphere and spectrally corrected measurements with a spectrograph and a Si CCD detector. [ 24 ]…”

Near surface defects: Cause of deficit between internal and external open‐circuit voltage in solar cells

“…This often leads to a deficit between internal and external V OC , that is, V OC,in – V OC,ex . The deficit can be observed in thin film solar cells such as Cu(In,Ga)(Se,S) 2 , 5,6 CdTe, 7 and perovskite 4,8,9 and is associated to interface recombination in the device 4,9–13 . Identifying the source of interface recombination and the underlying qFLs gradient is crucial for achieving higher efficiency in these devices and enabling better understanding of device physics.…”

“…The deficit can be observed in thin film solar cells such as Cu(In,Ga)(Se,S)2, 5,6 CdTe, 7 perovskite, 4,8,9 and is associated to interface recombination in the device. 4,[9][10][11][12][13] Identifying the source of interface recombination and the underlying qFLs gradient is crucial for achieving higher efficiency in these devices and enabling better understanding of device physics. The mismatch of the energy bands at interface between absorber and charge transport layer, 4,14,15 and Fermi-level pinning are the two commonly evoked models to explain why and more so, in which case interface recombination dominates.…”

Understanding Performance Limitations of Cu(In,Ga)Se2 Solar Cells due to Metastable Defects—A Route toward Higher Efficiencies

“…[ 11 ] Under these circumstances, no space charge region exists and the interpretation is simplified. The technique was recently applied to CIGS, [ 12 ] perovskite, [ 13 ] and CdTe, [ 14 ] where diode factors between 1 and 2 are observed. In the case of lowly doped perovskite, photoluminescence is always conducted in high‐injection conditions.…”