一方で|いっぽうで

1. 1Sets a contrasting aspect against the first one: 'while ~, on the other hand ~'.Ex. 2
2. 2Adds a further aspect alongside the first one, often with も: 'while ~, also ~'.Ex. 1

Neutral in register but slightly formal in tone. It is common in explanations, reports, essays and news, where two sides of a person, situation or policy are weighed against each other, and the second clause often contains も. Unlike 反面, which is limited to opposing sides, 一方で covers both contrast and simple addition.

• 反面N2「反面」 can only express two opposite qualities of the same subject, and in that case it is interchangeable with 「一方で」; it cannot contrast different subjects, nor can it be used to simply add another side. At the start of a sentence, 「その反面」 is also possible.
• 一方だN3「一方だ」 attaches only to the dictionary form of change verbs such as 「増える」 or 「悪化する」 and expresses one-way continuous change ('more and more...'), not contrast. Its mid-sentence form is the identical 「〜一方で」, so you must judge from the following clause whether it contrasts another side or leads into the result of the change.
• つつN2「つつ」 is written-style and attaches only to the continuative (renyōkei) form of verbs, expressing the same subject doing something 'while...' or 'knowing full well... yet...'. 「一方で」 attaches to various word classes, contrasts two sides of something, allows different subjects, and is stylistically neutral, so the two are generally not interchangeable.
• ながら / いながら / ながらもN3When 「ながら(も)」 is adversative, it means 'although... yet...', and nouns attach directly; 「一方で」 contrasts or supplements two aspects, requires 「である」 after nouns, and can also serve as a conjunctive. Note that 「ながら」 also has a simultaneous-action meaning.
• と同時にN2「と同時に」 can also mean two things happen at the same moment ('the moment...'), a use 「一方で」 does not have. When expressing two sides of something, the common phrasing is 「…であると同時に…も」, which stresses dual possession and is interchangeable with 「一方で」, though its contrastive nuance is weaker.

反面 also weighs two sides of one thing but is limited to opposing aspects and is not used for simple addition, while 一方だ means 'keep ~ing more and more', not 'on the other hand'. つつ (after verb stems) and ながら(も) (after verb stems, い-adjectives, な-adjective stems and nouns, as in 若いながらも and 子供ながらも) express concession ('although') or simultaneous actions, and と同時に stresses that two things happen or hold true at the same time.