Graded bandgap p-i-n diode solar cells are fabricated with a-(Si,Ge):H deposited by triode PECVD. These cells are optimized for low energy photon absorption, and are suitable for the bottom cell of a tandem cell structure. Initially, a series of a-(Si,Ge):H films with a wide range of germanium contents were grown to determine the alloyed material quality. The techniques used to characterize the material include electrical and optical measurements, as well as EDS, TEM and SEM analysis;Reproducibly high quality films were deposited using low chamber pressures (~20mTorr), high levels of hydrogen dilution, substrate temperatures around 300°C, and a negative substrate bias. The variation in material characteristics with germanium content is reported including dark- and photoconductivity, subgap absorption, Tauc gap and activation Oil, Gas, and Energy;A series of constant bandgap devices were also fabricated to determine the hole transport parameters, as well as the Urbach energy. A new computer model for the internal electric fields is used to analyze the device results. The model determines the quantum efficiency from the internal electric field profile and the hole transport parameters;The hole mobility lifetime product ([mu][tau]) has not been well characterized in a-(Si,Ge):H, and is quantified for the first time here using the computer model. The quantum efficiency values are accurately modeled by adjusting three parameters: the interface electric field, the p-layer absorption and the hole [mu][tau] product. Once the hole [mu][tau] product was determined for various levels of germanium content in the i-layer, an exponential relationship between the Urbach energy and [mu][tau] was found. This relationship is most likely due to the dispersive transport of holes in the valence band, and provides researchers with a new tool for determining the [mu][tau] product;The model was also used to guide the design of the optimized graded bandgap devices. A series of devices were then fabricated and tested. The devices with the best results had a multiple graded i-layer with a minimum bandgap energy of about 1.46 eV, and a thickness of about 0.4 [mu]m. The final optimized structure had an approximately 25% improvement in long wavelength quantum efficiency.