This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. We compare the dark current-voltage ͑IV͒ characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon ͑a-Si:H͒ p-i-n cells, organic bulk heterojunction ͑BHJ͒ cells, and Cu͑In, Ga͒Se 2 ͑CIGS͒ cells. All three device types exhibit a significant shunt leakage current at low forward bias ͑V Ͻ ϳ 0.4͒ and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage ͑I sh ͒, across all three solar cell types considered, is characterized by the following common phenomenological features: ͑a͒ voltage symmetry about V =0, ͑b͒ nonlinear ͑power law͒ voltage dependence, and ͑c͒ extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propose a space charge limited ͑SCL͒ current model for capturing all these features of the shunt leakage in a consistent framework and discuss possible physical origin of the parasitic paths responsible for this shunt current mechanism.
have the highest reported effi ciencies, [ 1 ] the manufacturing is still complex and costly. [ 2,3 ] There is a need for new materials growth, processing and fabrication techniques to address this major shortcoming of III-V-based photovoltaics. Signifi cant progress on this front has been made by the epitaxial lift-off and transfer technique developed for gallium arsenide (GaAs), [ 1,4,5 ] which allows for limited reuse of costly epitaxial substrates.Here we present an alternative approach using indium phosphide (InP) thin fi lms grown directly on metal substrates. InP has a direct band gap of 1.344 eV, which is optimal for maximum effi ciency in single junction solar cells. [ 6 ] Recently we developed the thin-fi lm vapor-liquid-solid (TF-VLS) growth technique to produce high optoelectronic quality InP absorber layers directly on molybdenum (Mo) substrates. In this implementation of the technique, a layer of indium (In) confi ned between a Mo substrate and a silica (SiO x ) cap is heated to a temperature at which In is a liquid. The SiO x cap serves to prevent In evaporation and dewetting of the liquid In. Then, phosphorus vapor is introduced which diffuses through the SiO x cap into the In liquid, causing precipitation of solid InP. The InP grows into a polycrystalline fi lm with ultra-large (>100 µm) lateral grain sizes. [ 7,8 ] This templated process extends the use of VLS for growth of structures beyond nanowires. [9][10][11] The The design and performance of solar cells based on InP grown by the nonepitaxial thin-fi lm vapor-liquid-solid (TF-VLS) growth technique is investigated. The cell structure consists of a Mo back contact, p -InP absorber layer, n -TiO 2 electron selective contact, and indium tin oxide transparent top electrode. An ex situ p -doping process for TF-VLS grown InP is introduced. Properties of the cells such as optoelectronic uniformity and electrical behavior of grainboundaries are examined. The power conversion effi ciency of fi rst generation cells reaches 12.1% under simulated 1 sun illumination with open-circuit voltage ( V OC ) of 692 mV, short-circuit current ( J SC ) of 26.9 mA cm −2 , and fi ll factor (FF) of 65%. The FF of the cell is limited by the series resistances in the device, including the top contact, which can be mitigated in the future through device optimization. The highest measured V OC under 1 sun is 692 mV, which approaches the optically implied V OC of ≈795 mV extracted from the luminescence yield of p -InP.Figure 4. a) Calculated equilibrium band diagram of the top surface region of the device. b) J -V measurements for a cell under simulated 1 sun illumination (solid line) and in the dark (dotted line). Device parameters were V OC of 692 mV, J SC of 26.9 mA cm −2 , FF of 65%, and power conversion effi ciency of 12.1%. Cell area was 0.5 × 0.5 mm 2 . c) Corresponding EQE and 1-R curves.
Cu2ZnSn(S,Se)4 (CZTSSe) solar cells typically exhibit high short-circuit current density (Jsc), but have reduced cell efficiencies relative to other thin film technologies due to a deficit in the open-circuit voltage (Voc), which prevent these devices from becoming commercially competitive. Recent research has attributed the low Voc in CZTSSe devices to small scale disorder that creates band tail states within the absorber band gap, but the physical processes responsible for this Voc reduction have not been elucidated. In this paper, we show that carrier recombination through non-mobile band tail states has a strong voltage dependence and is a significant performance-limiting factor, and including these effects in simulation allows us to simultaneously explain the Voc deficit, reduced fill factor, and voltage-dependent quantum efficiency with a self-consistent set of material parameters. Comparisons of numerical simulations to measured data show that reasonable values for the band tail parameters (characteristic energy, capture rate) can account for the observed low Voc, high Jsc, and voltage dependent collection efficiency. These results provide additional evidence that the presence of band tail states accounts for the low efficiencies of CZTSSe solar cells and further demonstrates that recombination through non-mobile band tail states is the dominant efficiency limiting mechanism.
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