Previously, an innovative way to reduce rear interface recombination in Cu(In, Ga)(S, Se)(2) (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu-2(Zn, Sn)(S, Se)(4) (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (V-OC; +17% rel.), short-circuit current (J(SC); +5% rel.), and fill factor (FF; +9% rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32% rel. is obtained for the rear passivated cells.

The benefits of gallium (Ga) grading on Cu(In, Ga) Se-2 (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive-level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free-carrier densities in the CIGS absorber, and their impact on the cell's open-circuit voltage V-oc. The parameter most constraining the cell's Voc is found to be the deep-defect density close to the space charge region (SCR ). In ungraded devices, high deep-defect concentrations (4.2 x 1016 cm(-3)) were observed near the SCR, offering a source for Shockley Read-Hall recombination, reducing the cell's Voc. In graded devices, the deep-defect densities near the SCR decreased by one order of magnitude (2.5 x 1015 cm(-3)) for back surface graded devices, and almost two orders of magnitude (8.6 x 1014 cm(-3)) for double surface graded devices, enhancing the cell's Voc. In compositionally graded devices, the free-carrier density in the absorber's bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band.

Atomic layer deposited (ALD) Al2O3 films on Cu(In, Ga)Se-2 (CIGS) surfaces have been demonstrated to exhibit excellent surface passivation properties, which is advantageous in reducing recombination losses at the rear metal contact of CIGS thin-film solar cells. Here, we report, for the first time, experimentally extracted electronic parameters, i.e. fixed charge density (Q(f)) and interface-trap charge density (D-it), for as-deposited (AD) and post-deposition annealed (PDA) ALD Al2O3 films on CIGS surfaces using capacitance-voltage (C-V) and conductance-frequency (G-f) measurements. These results indicate that the AD films exhibit positive fixed charges Q(f) (approximately 10(12) cm(-2)), whereas the PDA films exhibit a very high density of negative fixed charges Q(f) (approximately 10(13) cm(-2)). The extracted D-it values, which reflect the extent of chemical passivation, were found to be in a similar range of order (approximately 10(12) cm(-2) eV(-1)) for both AD and PDA samples. The high density of negative Q(f) in the bulk of the PDA Al2O3 film exerts a strong Coulomb repulsive force on the underlying CIGS minority carriers (n(s)), preventing them to recombine at the CIGS/Al2O3 interface. Using experimentally extracted Q(f) and D-it values, SCAPS simulation results showed that the surface concentration of minority carriers (n(s)) in the PDA films was approximately eight-orders of magnitude lower than in the AD films. The electrical characterization and estimations presented in this letter construct a comprehensive picture of the interfacial physics involved at the Al2O3/CIGS interface. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

An optimized test structure to study rear surface passivation in Cu(In,Ga)Se-2 (CIGS) solar cells by means of photoluminescence (PL) is developed and tested. The structure - illustrated in the abstract figure - is examined from the rear side. To enable such rear PL assessment, a semi-transparent ultrathin Mo layer has been developed and integrated in place of the normal rear contact. The main advantages of this approach are (i) a simplified representation of a rear surface passivated CIGS solar cell is possible, (ii) it is possible to assess PL responses originating close to the probed rear surface, and (iii) a stable PL response as a function of air exposure time is obtained. In this work, PL measurements of such structures with and without rear surface passivation layers have been compared, and the measured improvement in PL intensity for the passivated structures is associated with enhanced CIGS rear interface properties. [GRAPHICS] Transmission electron microscope (TEM) bright field cross-section image of the rear illuminated test structure fabricated for PL characterization.

Previously, an innovative way to reduce rear interface recombination of Cu(In,Ga)(S,Se)(2) (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu-2(Zn,Sn)(S,Se)(4) (CZTSSe) cells, to demonstrate its potential for other thin-film technologies. Therefore, ultra-thin CZTS cells with an Al2O3 rear surface passivation layer having nano-sized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open circuit voltage (V-OC; +49%(rel.)) or short circuit current (J(SC); +17%(rel.)), compared to corresponding unpassivated cells. Hence, a promising efficiency improvement of 52%(rel.) is obtained for the rear passivated cells.

This study deals with potential-induced degradation (PID) of Cu(In,Ga)Se-2-based solar cells and different approaches to subsequent recovery of efficiency. Three different recovery methods were studied: 1) etch recovery, 2) accelerated recovery, and 3) unaccelerated recovery. After being completely degraded, the solar cells with CdS buffer layers recovered their efficiencies at different rates, depending on the method which was used. On the other hand, if Zn(O,S) was used as a buffer layer instead of CdS, the recovery rate was close to zero. The buffer layer type clearly influenced the sodium distribution during PID stressing and recovery, as well as the possibilities for recovery of the electrical performance.

An innovative rear contacting structure for copper indium gallium (di) selenide (CIGS) thin-film solar cells is developed in an industrially viable way and demonstrated in tangible devices. The idea stems from the silicon (Si) industry, where rear surface passivation layers are combined with micron-sized local point contacts to boost the open-circuit voltage (V_OC) and, hence, cell efficiency. However, compared with Si solar cells, CIGS solar cell minority carrier diffusion lengths are several orders lower in magnitude. Therefore, the proposed CIGS cell design reduces rear surface recombination by combining a rear surface passivation layer and nanosized local point contacts. Atomic layer deposition of Al₂O₃ is used to passivate the CIGS surface and the formation of nanosphere-shaped precipitates in chemical bath deposition of CdS to generate nanosized point contact openings. The manufactured Al₂O₃ rear surface passivated CIGS solar cells with nanosized local rear point contacts show a significant improvement in V_OC compared with unpassivated reference cells.