Reducing absorber layer thickness below 500 nm in regular Cu(In,Ga)Se₂ (CIGS) solar cells decreases cell efficiency considerably, as both short‐circuit current and open‐circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano‐sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano‐sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano‐sized point contact openings. Evaporation of MgF₂ coated with a thin atomic layer deposited Al₂O₃ layer, or direct current magnetron sputtering of Al₂O₃ are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al₂O₃/CIGS rear interface. (MgF₂/)Al₂O₃ rear surface passivated ultra‐thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385 nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells. © 2014 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.

For the first time, a novel rear contacting structure for copper indium gallium (di)selenide (CIGS) thin film solar cells is discussed theoretically, developed in an industrially viable way, and demonstrated in tangible devices. The proposed cell design reduces back contacting area by combining a rear surface passivation layer and nano-sized local point contacts. Atomic layer deposition (ALD) of Al2O3 is used to passivate the CIGS surface and the formation of nano-sphere shaped precipitates in chemical bath deposition (CBD) of CdS to generate point contact openings. The Al2O3 rear surface passivated CIGS solar cells with nano-sized local rear point contacts show a significant improvement in open circuit voltage (VOC) compared to unpassivated reference cells. Comparing the passivated devices to solar cell capacitance simulator (SCAPS) modeling indicates that this increase is attributed to a decrease in rear surface recombination of a few orders.Paid Open AccessPercIG

Robust evidences are presented showing that the Raman mode around 250 cm−1 in the Sb2Se3 thin films does not belong to this binary compound. The laser power density dependence of the Raman spectrum revealed the formation of Sb2O3 for high values of laser intensity power density excitation under normal atmospheric conditions. To complement this study, the Sb2Se3 films were characterized by x-ray diffraction during in situ annealing. Both these measurements showed that the Sb2Se3 compound can be replaced by Sb2O3. A heat-assisted chemical process explains these findings. Furthermore, Raman conditions required to perform precise measurements are described.

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.

Recently, Cu(In,Ga)Se ₂ (CIGS) solar cells have achieved 21% world-record efficiency, partly due to the introduction of a postdeposition potassium treatment to improve the front interface of CIGS absorber layers. However, as high-efficiency CIGS solar cells essentially require long diffusion lengths, the highly recombinative rear of these devices also deserves attention. In this paper, an Al ₂ O ₃ rear surface passivation layer with nanosized local point contacts is studied to reduce recombination at the standard Mo/CIGS rear interface. First, passivation layers with well-controlled grids of nanosized point openings are established by use of electron beam lithography. Next, rear-passivated CIGS solar cells with 240-nm-thick absorber layers are fabricated as study devices. These cells show an increase in open-circuit voltage (+57 mV), short-circuit current (+3.8 mA/cm ² ), and fill factor [9.5% (abs.)], compared with corresponding unpassivated reference cells, mainly due to improvements in rear surface passivation and rear internal reflection. Finally, solar cell capacitance simulator (SCAPS) modeling is used to calculate the effect of reduced back contact recombination on high-efficiency solar cells with standard absorber layer thickness. The modeling shows that up to 50-mV increase in open-circuit voltage is anticipated.

Atomic layer deposited (ALD) Al₂O₃ films on Cu(In,Ga)Se₂ (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 10¹² cm⁻²), whereas the PDA films exhibit a very high density of negative fixed charges Q_f (approximately 10¹³ 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 10¹² 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.