Identification of Defect Levels in Copper Indium Diselenide (CuInSe2) Thin Films via Photoluminescence Studies

Abstract

Photoluminescence (PL) spectroscopy has been used to study the defect levels in thin film copper indium diselenide (CuInSe2, CIS) which we are developing as the absorber layer for the bottom cell of a monolithically grown perovskite/CuInSe2 tandem solar cell. Temperature and laser power dependent PL measurements of thin film CIS for two different Cu/In ratios (0.66 and 0.80) have been performed. The CIS film with Cu/In = 0.80 shows a prominent donor-to-acceptor peak (DAP) involving a shallow acceptor of binding energy ∼22 meV, with phonon replica at ∼32 meV spacing. In contrast, PL measurement of CIS film for Cu/In = 0.66 taken at 20 K exhibited an asymmetric and broad PL spectrum with peaks at 0.845 eV and 0.787 eV. Laser intensity dependent PL revealed that the observed peaks 0.845 eV and 0.787 eV shift towards higher energy (aka j-shift) at ∼11.7 meV/decade and ∼ 8 meV/decade with increase in laser intensity respectively. The asymmetric and broad spectrum together with large j-shift suggests that the observed peaks at 0.845 eV and 0.787 eV were related to band-to-tail (BT) and band-to-impurity (BI) transition, respectively. Such a band-tail-related transition originates from the potential fluctuation of defect states at low temperature. The appearance of band related transition in CIS film with Cu/In = 0.66 is the indicator of the presence of large number of charged defect states.

Charge Compensating Defects in Methylammonium Lead Iodide Perovskite Suppressed by Formamidinium Inclusion

“…In contrast, the defect peak at 1.46 eV blue-shifted with an increase in laser intensity at 16.6 meV/decade. Such a large blue-shift (aka, j -shift) is the characteristic of a band-tail (BT) related transition, which is widely present in copper chalcogenide semiconductors (e.g., CuInSe 2 and Cu 2 ZnSnS 4 ) possessing a large number of charge compensating defect states. − In highly compensating semiconductors consisting of similar amounts of donor and acceptor defects, electrons from the donor sites recombine with holes localized at the acceptor sites, leading to the coexistence of positively charged donors and negatively charged acceptors at various locations. As a result, spatially distributed charged regions establish the fluctuating potential of the valence band and conduction band edges .…”

Determination of Electronic Loss in Cu(In,Ga)Se2 Solar Cell by Spectroscopic Ellipsometry Analysis and External Quantum Efficiency Simulation

“…Study of degradation of CIGS, the optical properties and band gap of CIS/CIGS and Urbach tail energies were presented previously [7,20] and these results were applied to design the perovskite tandems [21,22]. Before studying CIGS, several studies were made for the CIS (CIGS: x = 0.0) including optical hall effect, structural and optical studies, device simulation and characterizations by different methods [6][7][8][23][24][25][26][27][28].…”