Solar cells convert solar energy directly into electricity and are attractive contribute to the increasing energy demand of modern society. Commercial mono-crystalline silicon based devices are infiltrating the energy market but their expensive, time and energy consuming production process necessitates alternative semiconductor materials. Recently advances in the field of polymer based solar cells have resulted in devices that feature power conversion efficiencies over 7% which approaches the threshold for successful commercialization. A fundamental tradeoff between current and voltage limits the ultimate power conversion efficiency in single junction solar cells because photons with energies larger than the band gap will lose their excess energy via thermal equilibration and photons with energies smaller than the band gap cannot be absorbed. In a tandem solar cell, a wide band gap subcell will reduce the thermalization losses of high energy photons and a small band gap subcell will lower the transmission losses of low energy photons. For efficient operation of the tandem, these subcells must be combined via a transparent intermediate contact consisting of an electron and hole transporting layer that enables a good physical separation of the two photoactive layers and a good electrical connection without voltage loss. This thesis aims to fabricate, optimize, and characterize polymer tandem solar cells with all layers processed from solution. Solution based techniques (e.g. printing) are commercially more interesting than evaporation or sputter steps in vacuum because these techniques are less time and energy consuming and eventually allow for high speed roll-to-roll production. The main challenge is to find materials that possess optimized optical and electronic properties for efficient operation and that can be processed into thin multilayer stacks from solution in such a way that each successive layer is deposited without disrupting the integrity of underlying layers. As a first step towards polymer tandem solar cells a transparent electron transporting layer has been developed that can be processed from solution (Chapter 2). It was found that a ZnO layer, deposited in the form of nanoparticles from acetone, can be employed. ZnO fulfills all requirements for an electron transporting layer and acetone is one of the few solvents that is innocuous to the underlying active layer and provides sufficient wetting to enable the formation of thin, closed, and transparent layers. Incorporating a ZnO layer in solar cells between the active layer and the reflective electrode led to working devices for active layers consisting of various polymers mixed with a fullerene derivative. In fact, insertion of the thin ZnO layer even led to an improved current density. Optical modeling showed that this beneficial effect can be attributed to a redistribution of the optical electric field inside the device that shifts the position of the maximum optical field into the active layer and increases the absorption of light. ZnO works as an optical spacer. This is especially interesting for polymer solar cells where the optimal layer thickness is limited by to a low charge carrier mobility. For the completion of the recombination layer, a hole transporting layer is needed. Because of its acidic nature, the commonly used PEDOT:PSS dispersion is detrimental for the ZnO layer. In Chapter 3 it is shown, however, that a pH neutral PEDOT can be used successfully as a hole transporting layer to fabricate solution processed polymer multiple junction solar cells. Contact problems at the ZnO/pH neutral PEDOT interface were resolved by photodoping of the ZnO layer. This increases the concentration of mobile electrons in the ZnO and creates an Ohmic contact between ZnO and pH neutral PEDOT. The open-circuit voltage of multiple junction solar cells increased from 1.57 to 2.19 and 3.58 V for two, three and six active layers. To make an efficient tandem cell, efficient wide and small band gap subcells must be combined. Chapter 4 describes the optimization of a small band gap cell based in a novel alternating copolymer of diketopyrrolopyrrole and quarterthiophene units. The morphology of the active layer and the performance of the solar cell were optimized by varying the processing conditions. Power conversion efficiencies of 3.6% and 4.0% were obtained in combination with C60 and C70 derivatives as acceptors by processing from a mixed chloroform/o-dichlorobenzene solution to ensure the correct balance between solubility and aggregation of polymer chains. In Chapter 5 it is demonstrated how a combined analysis of the optical absorption and electrical characteristics of the individual wide and small band gap single junction subcells can be used to identify the optimum device layout of the corresponding tandem cell. In contrast to existing views, matching the photocurrents of the subcells is not the best criterion for optimum performance, because the short-circuit current of a polymer tandem cell can exceed that of the current-limiting subcell. Using the new methodology, a solution processed polymer tandem cell with a power conversion efficiency of 4.9% was made, higher than the efficiency of the optimized single layer devices. This experimental result agrees well with the predicted value, indicating that this technique represents a universal method to improve the efficiency of future tandem solar cells. The current density generating capacities of the subcells were unmatched proving that the current-limiting subcell can be assisted by the other subcell in a polymer tandem solar cell. The strong optical and electrical interplay of the two subcells has important consequences for the accurate characterization of the tandem cell, especially with respect to measuring the external quantum efficiency. Chapter 6 describes a new procedure to accurately characterize two-terminal polymer tandem solar cells. The spectral response measurement of polymer tandem solar cells is complicated by sub-linear light intensity dependence and field-assisted current generation, requiring the use of an optical and an electrical bias. The required magnitude of optical and electrical biases was determined using single junction "dummy" cells, identical to the tandem subcell, and by determining the absorption spectra of the active layers in the stack obtained via optical modeling. Finally, the current density to voltage characteristics of the subcells of polymer tandem solar cells—permitting a better understanding of the functioning of the tandem cell—has been explored via two techniques (Chapter 7). First, an auxiliary electrode in between the electron and hole transporting layer but situated just outside of the photoactive area enabled making a three-terminal device structure without disturbing the optical electric field in the stack. This auxiliary electrode can be used for determining the bias offset between the recombination layer and either terminal electrode to provide the subcell characteristics. As an alternative method, an electrical bias sweep in the spectral response measurement of the subcells results in the current density to voltage curves of the subcells after convolution of the spectral response with the AM1.5G solar spectrum. The results of the different methods were verified and validated by cross-checking. In conclusion, the research described in the thesis has established new methods for fabricating, optimizing, and characterizing polymer tandem solar cells that can be processed from solution. The methods that have been developed are general and can be used to manufacture efficient tandem structures based on a detailed knowledge of the characteristics of single junction cells. Combined with the recent advances in the efficiency of organic solar cells based on new materials, the prospects for efficient polymer tandem solar cells are excellent
