The non-toxic and wide bandgap material TiO2 is explored as an n-type buffer layer on p-type Cu(In,Ga)Se2 (CIGS) absorber layer for thin film solar cells. The amorphous TiO2 thin film deposited by atomic layer deposition process at low temperatures shows conformal coverage on the CIGS absorber layer. Solar cells from non-vacuum deposited CIGS absorbers with TiO2 buffer layer result in a high short-circuit current density of 38.9 mA/cm2 as compared to 36.9 mA/cm2 measured in the reference cell with CdS buffer layer, without compromising open-circuit voltage. The significant photocurrent gain, mainly in the UV part of the spectrum, can be attributed to the low parasitic absorption loss in the ultrathin TiO2 layer (~10 nm) with a larger bandgap of 3.4 eV compared to 2.4 eV of the traditionally used CdS. Overall the solar cell conversion efficiency was improved from 9.5% to 9.9% by substituting the CdS by TiO2 on an active cell area of 10.5 mm2. Optimized TiO2/CIGS solar cells show excellent long-term stability. The results imply that TiO2 is a promising buffer layer material for CIGS solar cells, avoiding the toxic CdS buffer layer with added performance advantage.

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.