Wide bandgap Cu(In,Ga)Se₂ solar cells with improved energy conversion efficiency

Abstract

We report on improvements to the energy conversion efficiency of wide bandgap (Eg > 1.2 eV) solar cells on the basis of CuIn1−xGaxSe2. Historically, attaining high efficiency (>16%) from these types of compound semiconductor thin films has been difficult. Nevertheless, by using (a) the alkaline‐containing high‐temperature EtaMax glass substrates from Schott AG, (b) elevated substrate temperatures of 600–650 °C, and (c) high vacuum evaporation from elemental sources following National Renewable Energy Laboratory's three‐stage process, we have been able to improve the performance of wider bandgap solar cells with 1.2 < Eg < 1.45 eV. The current density–voltage (J–V) data we present includes efficiencies >18% for absorber bandgaps of ~1.30 eV and efficiencies of ~16% for bandgaps up to ~1.45 eV. In comparing J–V parameters in similar materials, we establish gains in the open‐circuit voltage and, to a lesser degree, the fill factor value, as the reason for the improved performance. The higher voltages seen in these wide gap materials grown at high substrate temperatures are due to reduced recombination. We establish the existence of random and discrete grains within the CIGS absorbers that yield limited or no generation/collection of minority carriers. We also show that interfacial recombination is the main mechanism limiting additional enhancements to open‐circuit voltage and therefore performance. Solar cell results, absorber materials characterization, and experimental details and discussion are presented. Copyright © 2012 John Wiley & Sons, Ltd.

Influence of Chemical Composition Heterogeneity on the Spectral Position of the Fundamental Absorption Edge of Cu(In, Ga)Se2 Solid Solutions

“…This signifi cantly expanded the technical potential of highly effi cient solar cells compared with the previously achieved η ~ 19.9% for x = 0.3 and E g ~ 1.1-1.2 eV [4]. In particular, η ~ 16 and >18% for E g ~ 1.45 and 1.30 eV for thin-fi lm CIGS solar cells [1]. Therefore, further progress in fabricating CIGS solar cells may be related to obtaining more detailed information on their optical properties (absorption, refl ectance, luminescence, etc.)…”

“…The most signifi cant practical result was the fabrication of solar cells of CuIn 1-x Ga x Se 2 (CIGS) solid solutions with effi ciencies η ~ 19.0-20.3% [1-6]. These effi ciencies are some of the highest for known thin-fi lm semiconductor solar-energy collectors [1,3,4]. In particular, the most similar competitive solar cells that represent an alternative to those based on CIGS solid solutions are based on simpler semiconductors and have lower effi ciencies, e.g., CdTe, η ~ 18.3%; amorphous α-Si:H, η ~ 16.1%; and nanocrystalline Si (nc-Si), η ~ 10.1% [6].…”

To pump the thermal energy from the dark world

“…[20,21] Some current solar cells are also made of GaAs, InP, GaInP, CIS, CIGS, CZTS, CdTe, etc. [22][23][24][25][26][27][28] GaAs thin-film cell, with a bandgap of 1.4 eV, demonstrates a high efficiency of 28%. [29,30] Dye-sensitized solar cells [31,32] and organic solar cells [33,34] also develop quickly.…”

Ultrafast pump-probe reflectance spectroscopy: Why sodium makes Cu(In,Ga)Se2 solar cells better

“…Potentially, reduced crystal lattice distortion leads to a minimum in defect concentration [23]. Furthermore, grain size and subsequently grain boundary area are sensitive to Ga/(In+Ga) [7,8,24]. Therefore, concomitant changes and interdependencies between Na, O incorporation, Ga/(In+Ga), and grain boundary area and activity present a challenge to unraveling the source of improvements made to the CIGS absorber and solar cell.…”

“…Improvements made over the years to increase CIGS solar cell efficiency include bandgap grading, which relies on liquid-assisted growth by the three-stage coevaporation method [1]: Na diffusion out of a soda-lime glass [2], postdeposition treatment (PDT) with NaF impacting grain boundary passivation [3], PDT with KF [4,5], and optimization of Ga/(In+Ga) composition [5][6][7][8]; however, improvements require substantially further study. In light of the important role that Na plays in CIGS layers, several groups have examined the growth and performance of CIGS layers by systematically varying Na content [9][10][11][12].…”