Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.
This review summarizes the current status of Cu(In,Ga)(S,Se) 2 (CIGS) thin film solar cell technology with a focus on recent advancements and emerging concepts intended for higher efficiency and novel applications. The recent developments and trends of research in laboratories and industrial achievements communicated within the last years are reviewed, and the major developments linked to alkali post deposition treatment and composition grading in CIGS, surface passivation, buffer, and transparent contact layers are emphasized. Encouraging results have been achieved for CIGS-based tandem solar cells and for improvement in low light device performance. Challenges of technology transfer of lab's record high efficiency cells to average industrial production are obvious from the reported efficiency values. One section is dedicated to development and opportunities offered by flexible and lightweight CIGS modules.
Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.
Absorbers for planar perovskite solar cells are often prepared by two‐step deposition methods, where a thick compact PbI2 layer is deposited followed by conversion to perovskite using CH3NH3I solution. The surface of the precursor layer quickly reacts with the CH3NH3I solution, which hinders further diffusion of CH3NH3I into the layer, consequently leading to significant amounts of residual PbI2 at room temperature. Here, we report a novel concept that employs a porous nanostructured PbI2 layer consisting of nanoplates to rapidly prepare single phase perovskite layer. The non‐compact nanoplate morphology is achieved in a controllable manner by thermal evaporation of PbI2 on TiO2‐coated FTO substrate and allows easy penetration of CH3NH3I solution into the whole PbI2 layer, thus facilitating fast and complete conversion. The amount of PbI2 residual can be controlled by varying the CH3NH3I concentration. The growth of the nanoplates is governed by the intrinsic crystallographic structure of the deposited material, crystal characteristics of the underlying substrate, and deposition method. The introduced process enables planar perovskite solar cells with efficiency of 8.6% measured at maximum power point. This work opens a new route for rapid preparation of other three‐dimensional organic–inorganic hybrid perovskites by rational tailoring the metal halide morphology.
The importance of Cu for CdTe solar cell absorber doping has been increasingly recognized in recent years. Currently different models are being discussed how to understand the case of Cu Cd substitutional doping in polycrystalline CdTe solar cells. In this work, an understanding is developed, which is based on a low concentration deep acceptor doped CdTe layer (N a $ 5 Â 10 14 cm À3 ,E a $ 300 meV above the valence band). Despite their non-shallow nature, Cu Cd acceptors are fully or at least heavily (>30%) ionized. The low hole concentration in CdTe ($1 Â 10 14 cm À3) originates directly from low Cu solubility in CdTe bulk material and is not caused by partial ionization or compensation as proposed by earlier models. The three to four orders of magnitude difference between bulk acceptor concentration and average Cu concentration in polycrystalline CdTe is attributed to grain boundary segregation of Cu. Our model is derived from substrate and superstrate CdTe solar cell measurements, controlled CdTe doping and quenching, Hall Effect measurements of CdTe films, numerical and analytical calculations, and a broad literature survey. Based on these results, routes to improve the conversion efficiency of CdTe solar cells are discussed. V
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