The presentation will briefly summarize recent progress on nanocrystal-ink based routes to CIGS. However, given the terawatt scale of future energy needs, the most promising future photovoltaic materials should be Earth abundant with their primary mineral resources distributed across several geographic regions and their supply chains robust to reduce concerns of price volatility. In addition, the process of forming the solar cell should be scalable, low-cost, and not utilize dangerous or toxic materials. The strongest initial candidate appears to be Cu₂ZnSnS₄ (CZTS). Up until 2009, CZTS thin film solar cells were synthesized primarily by evaporating or sputtering metals (Cu, Zn, & Sn) followed by sulfurization. More recently, two potentially low-cost high-throughput approaches have been demonstrated that form the quaternary or pentenary chalcogenide directly from solution-phase processes. One is based on first synthesizing multinary sulfide nanocrystals and then sintering them to form a dense layer. The other approach utilizes molecular precursors dissolved in hydrazine. Both new approaches reach their highest device efficiencies by incorporating Se to form Cu₂ZnSn(S_x,Se_1-x)₄ devices, and each has yielded substantially higher efficiency devices than the best vacuum deposited absorbers. The hydrazine route has yielded the most efficient CZTS-based devices thus far. The presentation will focus on our recent progress in CZTS-based nanocrystal-ink devices. In particular, we have shown that germanium may be alloyed with CTZS (at least up to Ge/(Sn+Ge) ratios of 0.7) to form Cu₂Zn(Sn,Ge)S₄ nanocrystals that have an increased bandgap [1]. The defect chemistry is serendipitous, and allows for at least 6.8% efficient devices at high germanium content. This exciting prospect may be used to create a back surface field and direct carriers in a similar manner to how gallium is used in high efficiency CIGS devices. In addition, we will report recent results on a scalable chemical route to Earth abundant element thin film solar cells by coating a solution of highly soluble, inexpensive, and commercially available precursors in an environmentally friendly non-toxic solvent to form device quality films (without using nanocrystals or hydrazine). [1] Ford, G.M., Guo, Q., Agrawal, R., and Hillhouse, H.W., “Earth Abundant Element Cu₂Zn(Sn_1-xGex)S₄ Nanocrystals for Tunable Band Gap Solar Cells: 6.8% Efficient Device Fabrication,” Chemistry of Materials 23 (10), 2626–2629 (2011).

Future terawatt-scale solar electricity requires lower photovoltaic (PV) module fabrication costs, reliance on more abundant materials and efficiency equal to or greater than current commercial technologies. Thin-film indium-based chalcopyrite devices have one of the fastest growth potentials and also highest efficiencies among thin-film PV technologies. Replacement of the scarce indium element is viewed as a strategic step towards unlimited growth of chalcogenide PV production beyond 100GWp/year. A strong candidate in this category is the kesterite family, including Cu2ZnSnS4 and Cu2ZnSn(S,Se)4 (CZTS and CZTSSe). We have developed kesterite solar cells operating in the 10% efficiency range. This performance is achieved by a simple low-cost liquid-based deposition approach based on chalcogenide solutions in combination with particle-based precursors that yield homogeneous CZTS and CZTSSe phases after thermal treatment. Here we will present recent advances in this field.

Copper zinc tin sulfide (Cu₂ZnSnS₄, CZTS) is a very promising material as a low cost absorber alternative to other chalcopyrite-type semiconductors based on Ga or In because it is only composed of abundant and economical elements. In addition, CZTS has a direct band-gap energy of 1.0~1.5 eV and large absorption coefficient over ~10⁴ cm^-1, which are similar to those of Cu(In,Ga)Se₂(CIGS) that is regarded as one of the most successful absorber materials. Typically, metal chalcogenide films such as CIGS and CZTS are deposited by vacuum process such as evaporation or sputtering. However, this vacuum deposition suffers from relatively low throughput, low material utilization, and difficulties associated with large-scale production. In this regard, solution-based deposition methods are being developed because they have advantages including suitability for large-area substrates, higher throughput, and more efficient materials usage. Various solution based approaches for producing absorber layer have been reported including sol-gel and nanocrystal dispersion, but they are still facing some limitations; sol-gel method is vulnerable to the contamination of carbon, oxygen, and other impurities from the precursor solution and inevitably leads to the formation of porous structure. Nanocrystal dispersion method needs the complex synthesis of nanocrystals. Recently, Todorov et al. reported the fabrication of CZTS thin film solar cells with 9.6% power conversion efficiency using a hydrazine based approach. However, hydrazine is a highly toxic and very unstable compound that requires extreme caution during handling and strorage. Furthermore, due to the reactive nature of this solvent, all processings for slurry and film preparations must be performed under inert atmospheric conditions and thus it would not be easily adapted for large-scale solar cell fabrication. With these considerations, it is highly desirable to develop a robust, easily scalable and relatively safe solution-based process for the fabrication of high quality CZTS absorber layer. Here, we devise the air-stable hybrid solution-particle approach for the fabrication of dense CZTS absorber layer. Our air-stable ink comprises of commercially available powder mixture of Zn, Sn, and S dispersed in an environmentally benign solvent in which Cu and Zn precursors are dissolved. The metal precursor solution is vulcanized by sulphur element to create a rubberlike polymer, providing the viscosity and wetting that allows us to prepare highly stable hybrid ink containing heavy metallic powders. Our readily achievable hybrid ink, without the involvements of complex particle synthesis and high toxic solvent, enables a convenient access to fabricate uniform, dense, contaminant-free, large-grained CZTS absorber layer. Our simple approach reported here will be the first step in realizing the low-cost and large-area solar cells with high efficiency.

Non toxic chemical routes that enable formation of high quality CuInSe2 thin films with high materials utilization are desired for low production cost of solar cells. We will present results on the effects of ultrasound on Cu,Se and In,Se elemental precursors with different solvents. Depending on the solvent, the reaction between these elemental precursors facilitates binary selenide phases. These crystalline phases are compared with the effective heat of formation model of Cu-Se and In-Se compounds. The formed binary selenide phases gave an exact match with the predicted effective heat of formation model. However, sonication of Cu,In and Se elemental particles for short times did not yield any single phase CuInSe2 due to the unfavored reaction pathways between CuSe2 and In4Se3 phases. Further annealing these binary phases led to the single phase formation of CuInSe2 at 350C. SEM studies revealed upon sonication above one micron sized Cu,In,Se elemental particles are broken to smaller particles during sonication and moreover upon completion of the reaction CuInSe2 nanocrystals are obtained.

CuIn(Se,S)2 and other chalcopyrite-based solar cells have received substantial research interest owing to their high optical absorption coefficient, a tunable band gap and demonstrated the highest power conversion efficiency among thin film solar cells. However, the use of vacuum technique in the preparation of absorber layers imposes hurdles in the the production of low cost and large area photovoltaic modules. Solution-based absorber deposition approaches offer an alternative to many of these issues, and have thus been extensively pursued for more than a decade. Recently, a hydrazine solution based processing method has demonstrated copper chalcopyrite based thin film solar cells with efficiencies of up to 13.6% despite the early developmental stage of this technique. In order to further improve this approached solar cells performance, deeper understanding on dissolving mechanism, the structure of molecular precursors, phase formation, defect physics and recombination mechanisms in such devices would be necessary. In this talk, we present the molecular structures present in solutions containing Cu2S, In2Se3, or mixtures of the two, and the CuIn(Se,S)2 phase formation after annealing the solution-processed precursor films. We also examine the effects of introducing an additional Cd ion solution soaking step into our device fabrication process on the defect energy levels and carrier concentration which in turn consistently improves the open circuit voltage of solar cell devices.

Solution-processed CuIn(S,Se)2 has shown considerable promise as an absorber material in the fabrication of high performance solar cells. However, it is crucial to indentify the primary factors limiting photovoltaic performance. In this study, photoluminescence spectroscopy is employed to investigate the carrier recombination in CuIn(S,Se)2 with emphasis on the effect of cadmium sulfide (CdS) deposition. In steady state photoluminescence, it has been found that CdS deposition (1) leads to an altered photoluminescence profile due to cadmium diffusion into the CuIn(S,Se)2 layer as well as defect passivation at the absorber surface. (2) CdS deposition helps to protect the CuIn(S,Se)2 film from chemical degradation, and also (3) the shallow defects in CuIn(S,Se)2 film at roughly 31 meV above the valence band and with density about 1015cm-3 are estimated through temperature-dependent photoluminescence measurements. These effects have been studied quantitatively, and the results are presented along with suggestions for how future devices can be optimized with respect to material parameters and fabrication strategies.

The use and characterisation of solution processable inorganic semiconductors is highly relevant to the development of low-cost electronic devices such as solar cells. In this paper we will report on two aspects of our work in this area. Firstly, we will describe our layer-by-layer process that enables the fabrication of totally solution processable solar cells from inorganic nanocrystal inks in air at temperatures as low as 300 degrees Celsius. Focusing on a CdTe/ZnO thin-film system we report solar cells that achieve power conversion efficiencies of over 7% with greater than 90% internal quantum efficiency (IQE). We will describe the characterisation of the devices and, through capacitance-voltage measurements, demonstrate that the CdTe layer is fully depleted which enables charge carrier collection to be maximized. Finally, we also discuss the use of photoelectron spectroscopy in air (PESA) to investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS and PbSe semiconductor quantum dots (QDs). We will compare the results to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k-p theory and state-of-the-art atomistic semi-empirical pseudopotential modelling.

We report a highly efficient hybrid nanostructured solar cell consisting of a densely-packed TiO2 film; a PbS nanocrystal film; a hole transporting polymer layer (HTL); and metal contacts. The TiO2 film was spray deposited on an ITO coated glass slide. Subsequently, a PbS nanocrystal film was deposited, layer-by-layer, on the TiO2 film to establish a planar donor-acceptor heterojunction. Two device geometries were fabricated on this structure by subsequently depositing either: i) a single Au contact layer or ii) a HTL followed by the Au contact layer. Drastic increases in the photocurrent, and stability in ambient air, was observed in the device with the HTL, relative to the device without the HTL. The device with the HTL layer had a power conversion efficiency of more than 5% under AM1.5G (100 mW/cm2). Moreover, the external quantum efficiencies of this device was very high (~90% in the visible range [400~600 nm] and ~30% in the near infrared range). The dependence of photovoltaic performance of the device on the thickness of the PbS nanocrystal film and the TiO2 film will be presente