Postdeposition treatments (PDTs) of chalcopyrite absorbers with alkali fluorides have contributed to improving the efficiency of corresponding solar cell devices. However, cells prepared with PDTs also tend to exhibit nonideal current–voltage (J–V) characteristics especially at low temperatures. These include blocking of the forward diode current, saturation of the open‐circuit voltage with respect to temperature, a discrepancy between dark and Jsc (Voc) characteristics, and a crossover between dark and light J–V curves. These are typical observations while measuring the temperature‐dependent J–V characteristics. Herein, the influence of electronic material parameters on the blocking of the current across the heterojunction in numerical simulations is reported. It is shown that a low‐doped ZnO window layer, acceptor defects at the CdS/ZnO interface, or a high band offset at that interface lead to similar nonideal J–V characteristics, suggesting that the carrier density in the buffer layer is a crucial parameter for the current limitation. Connections between the effects of PDT previously reported in literature and the electronic material parameters considered in the numerical model are discussed to explain the nonideal J–V characteristics caused by the PDTs.

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In this contribution, the impact of thermal stress on Cu(In,Ga)Se 2 (CIGSe) thin film photovoltaic devices is investigated. The tolerance of such devices to high temperatures is of particular interest for processing transparent conductive oxides (TCOs) in order to further close the gap to the theoretical efficiency limit and for their potential use as bottom devices in tandem applications in order to overcome the theoretical efficiency limit of single junction solar cells. When CdS-buffered CIGSe high efficiency solar cells are subjected to thermal stress, elemental interdiffusion of Na and Cd between the absorber and the window layers as well as chemical reactions at the CIGSe/CdS interface result in a degraded power conversion efficiency (PCE). Here, we compare the degradation mechanisms of CdS and GaO x buffered CIGSe solar cells under thermal stress. A model explaining the observed degradation behaviors is proposed. Index Terms-Amorphous buffer layer, Cu(In,Ga)Se 2 (CIGSe) solar cells, CdS, thermal stability, GaO x. I. INTRODUCTION C u(In,Ga)Se 2 (CIGSe) thin film solar cells have attracted the attention of science and industry by attaining a maximum Manuscript

We present a comprehensive device model for Cu(In,Ga)Se 2 (CIGSe) thin-film solar cells based on numerical SCAPS-1D simulations. The model reproduces the experimentally determined current-voltage and capacitance-voltage characteristics of a Rb-free reference device, a sample that underwent an RbF-treatment, and a sample based on a CIGSe/RbInSe 2 -stack. According to this model, and in agreement with experimental findings, the main consequences of both Rb-conditionings are an increased doping-density and a defect passivation in the CIGSe as well as the formation of a photocurrent barrier at the hetero interface. With the numerical model established, fundamental aspects of the Rb-conditioning, e.g., the differentiation between its effect on bulk and interface recombination are discussed. Additionally, temperature dependent current-voltage analysis is employed in order to test the model's predictions regarding the interaction of Rb with an injection-current barrier at the back contact of the device. Both the simulation and the temperature dependent current-voltage measurements lead to the result that the RbF-PDT is increasing the height of this barrier, while the deposition of RbInSe 2 is decreasing it. Index Terms-Cu(In,Ga)Se 2 (CIGSe) solar cells, device simulations, RbF-postdeposition treatments (PDT), RbInSe 2 , SCAPS. I. INTRODUCTION D URING the last 9 years, postdeposition treatments (PDTs) of Cu(In,Ga)Se 2 (CIGSe) thin-films using heavy alkalifluorides led to several record efficiencies [1]-[4]. Thereby, most studies consistently report the main effects of these PDTs to be an increased carrier concentration of the CIGSe (p CIGSe ) [3], [5]-[7], improved minority carrier lifetime (τ n ) [3], [6], therefore, an increased open-circuit voltage (V OC

Due to their tunable bandgap energy, Cu(In,Ga)Se2 (CIGSe) thin‐film solar cells are an attractive option for use as bottom devices in tandem configurations. In monolithic tandem devices, the thermal stability of the bottom device is paramount for reliable application. Ideally, it will permit the processing of a top device at the required optimum process temperature. Here, we investigate the degradation behavior of chemical bath deposited (CBD) CdS‐buffered CIGSe thin‐film solar cells with and without Na incorporation under thermal stress in ambient air and vacuum with the aim to gain a more detailed understanding of their degradation mechanisms. For the devices studied, we observe severe degradation after annealing at 300°C independent of the atmosphere. The electrical and compositional properties of the samples before and after a defined application of thermal stress are studied. In good agreement with literature reports, we find pronounced Cd diffusion into the CIGS absorber layer. In addition, for Na‐containing samples, the observed degradation can be mainly explained by the formation of Na‐induced acceptor states in the TCO front contact and a back contact barrier formation due to the out‐diffusion of Na. Supported by numerical device simulation using SCAPS‐1D, various possible degradation models are discussed and correlated with our findings.